IT WAS AFTER LUNCH when young Jim Fallows returned to his quarters in the Executive Office Building and, if you will pardon the expression, he was still a little wet behind the ears.

His office is one of those large and overdecorated tombs of stillness in the EOB, where bright young men labor for President under awesomely high ceilings.

Fallow is bright and younger than most of them, a mere 27 years old, which is approximately 10 years older than he looks. Quite Young to become the chief speech writer for a President of the United States or to have your own fireplace.

To be fair about it, Fallows was damp only because he was out jogging on his lunch hour, doing a few miles around the national monuments.He showered afterwards in the recently liberated H.R. Haldeman memorial sauna" in the basement.

The Haldeman sauna is mildly relevant because Fallows is a journalist who has written perceptively as an outsider on the seductions of White House power, the private mores of our bureaucratic culture which corrupts so subtly and, perhaps, shapes political behaviour as deeply as issues or ideology. Now he's on the inside, experiencing the same stuff for real.

In that regard, Fallows mentions the sauna as a light token of what has changed at the White House. In former days, it was an exclusive place, where 15 or 20 Nixon centurious could sweat out their anxieties or whatever.

Now the sauna is inoperative and the Carter people have democratized it. The locker room is open to all employees regardless of rank, including White House women - though not to men and women at the same hours.

So some things do change, but Fallows is interested and willing to talk about some things which do not. Having written so much about how large egos behave pettily inside government, this bright young man proceeds to confess that, in the past month, he has caught himself doing the same. Judging office space in terms of its proximately to the Oval Office. Comparing salaries as a measure of status. Expanding the workload to keep up with the expanding staff.

"There was a time when I was going to have an office in the West Wing, then there was a time when I didn't," Fallos recalled. "And, for about three hours I was in a panic."

He found himself doing what other bright young strivers have done before him - getting on the phone and wangling, pleading, whatever. Then he realized how silly it was - and how predictably bureaucratic.

"With what I specificially have to do, when there's work to be done, I see everybody I need to see," he reminded himself. "And when there's not work to be done, I would gain nothing by loitering on the premises in hopes that Carter would round the ball and bump into me. That doesn't pay off especially with him."

It was equally ridiculous, Fallows allowed, to worry about his salary level and where it puts him in the new pecking order. At $37,800, he is now making more than twice what he ever earned previously (and will get still more when the federal pay raise comes through). Fallows expresses what went through his head:

"I've seen little flickerings of that feeling, deep in my heart, that instead of comparing how much I'm earning with either my needs or with my previous income, I compared it with the guy down the hall who is making a little more. I'm more valuable than he is. Why is this happening?"

The bureaucratic feeling of it, as Fallows says, was "knowing all the while what I was doing but not being able to help myself." Nonconformist Background

IF THESE SEEM like small matters, they are genuine expressions of the survival impulses generated inside the White House or any other [WORD ILLEGIBLE] organization, impulses which alter one behavior, even for people who knows better. [WORD ILLEGIBLE] competes with substance, very often replace it as the main objective. Given an excess of adrenalin and an absence of ethical values, the bright young men sometimes wind up in big black headlines as a John Dean or a Jeb Magruder.

Fallows knows all this, which is what makes him so interesting to watch, now that fortune has landed him so quickly in high places. He knows that, between Vietnam and Watergate, bright young men rising quickly in government have gotten a bad name. He knows why, because he had written on the subject frequently, usually for The Washington Monthly, a small but influential chronicle of the capital's working ethics (or lack thereof).

His own background is nonconformist, a budding career of dissent and critical examination of the established institutions.

At Harvard, Fallows was president of the Crimson in 1969, that tumultuous period of campus dissent when the student daily newspaper was widely denounced as "the pinko rag."

After a Rhodes scholarship, he came to Washington to work for Ralph Nader, co-authoring two Nader reports, including the controversial "Who Runs Congress?"

Afterward, Fallows offered his own tart critique of Nader reform - too shallow, unduly abrasive, founded on false expectations of citizen action.

As a young journalist in Washington, Fallows has tweaked a number of prominent noses - columist Joseph Kraft, Washington Post editor Benjamin Bradlee, novelist-editor Willie Morris, former LBJ aide Bill Moyers, novelist-critic Mary McCarthy, among others.

Fallows has gotten tweaked himself for some of his journalist adventures. The 1972 Nader paperback on Congress was attacked as a glossy rewrite of standard complaints which others had made of that institution - a "clip job," according to some critics. His analysis of McCarthy's reporting drew an upbraiding from The New York Review of Books where she is a regular contributor. Evaluating a Speech

JIM FALLOWS is not enraptured by the job he is doing now (heading a staff of six), despite the pay and glamor office and prestige.

"The thing which most speechwriters describe I've not felt - that flutter of the heart when the President of the United States utters your words," Fallows said.

The reason, he quickly added, is that Carter mostly utters his own words, not artful phrases cranked out by a speechwriter. For most occasions so far, the speechwriters have only prepared subject notes for Carter not texts, and this requires straight-ahead writing style with no ruffles or flourishes.

"I recognize full well," Fallows said, "That if Carter had the time, he'd prefer to have no speechwriters and just write everything himself, as he did his own inaugural address, and so one's role as 'puppeteer' is, well, one doesn't entertain those thoughts."

One speech which Fallows did work on was the President's "fireside chat" and he was also asked to evalute it, from two perspectives - as an insider on the White House staff and as he might have reviewed it on the outsides as a journalist.

From the inside: "By the standards we set forth, I. think it was a success, both as a matter of tone and as a matter of substance. It was unassuming, straightforward, non-rhetorical, trying to establish the fact that people needn't be suspect of Carter for his plainness and his apparent aversion to pomp . . . In content, it was trying to say, "This is what I told you I was going to do and here's how I'm trying to carry it out' . . . It was way too long, but that's a matter of succumbing too easily to the pressure to cram in extra stuff by people who want extra stuff crammed in."

From the outside: "I would have thought, sitting there getting ready to write some critique of it, that these people know what they're doing. Maybe it's one degree too cute. Maybe it's one degree too devoid of specifics. But they basically know what they're doing and they're trying to get their man across." "Rose Garden Rubbish"

AS A WRITER, Fallows has learned that political speeches are themselves limited vehicles for expression. He joined the Carter speechwriting staff last summer, looking for a chance to see the inside of politics, storing up insights for his future as a journalist. After the election, he sought a place on the White House issues staff under Stuart Eizenstat, but before the Inaugural, chief speechwriter Patrick Anderson left to return to writing novels and Carter abruptly offered Fallows the job.

The grapevine is still debating whether Anderson jumped or was pushed, with three or four competing versions circulating. Anderson has said that he concluded that his own work as a writer was more compelling to him at this point in his life; others whisper that there was presidential disenchartment involved.

In any case, Fallows was not angling for the job, by all accounts, including Anderson's. Carter, it is said, got along with the young man immediately in their limited contact during the campaign. Both are serious people.

Carter is also a longtime subscriber of The Washington Monthly which espouses a small-d democratic view of what government should be, fundementally in harmony with Carter's program and rhetoric (though it has done some critical numbers on the Carter administration's first days). Carter, it is said, read a sampler of Fallow's past articles and liked them.

"In my heart, I much prefer magazine writing," Fallows said, "because there's a limit to what sort of things you can explore in a political speech. It has to be immediately accessible to everyone who hears it. I discovered during the campaign you could almost never educate or teach through a speech, but instead you had to string together assertions which people would recognize immediately . . . It's a form whose value I recognized and I'm trying to perform as well as I can, but it's not in the long run something I find as satisfying as what I used to do."

Fallows does enjoy writing or rewriting the steady stream of presidential proclamations, messages and cover letters which are issued under Carter's name. One former White House writer dubbed this material the "Rose Garden rubbish," but Fallows says he likes the challenge of translating garbled and guarded bureaucratese into plain talk.

"Carter takes very seriously things which go out under his name which are not speeches," he said. Inside Politics

FALLOWS GREW UP in Redland., east of Los Angeles, the son of a prominent doctor. Unlike so many brisk young men scrambling upward in politics, Fallows says, he is not searching for a father figure. He already has one whom he loves and admires.

His service to Jimmy Carter, Fallows admits, began with a large measure of self-intertest. In the fall, as candidate Carter was slipping and criticism increased, Fallows found his own commitment deepening.

"I thought then," he said, "that the worst thing that could happen if Jimmy Carter becomes President is that nothing would happen, that is, he would not be able to get along with the Congress and so his various reforms would not get through, whereas the best thing that could happen is that there would be a fundamental change, in the order of Franklin Roosevelt's, in the way the public business is conducted."

As a boss, Carter is serious and businesslike and, as Fallows put it deftly, "doesn't squander his charm senselessly." On the other hand, he said, Carter does not demand "slavish attention" from subordinates, in the manner of some predecessors.

"He's an extremely intelligent man," Fallows said. "He's also accustomed to being one of the most intelligent people around and so has the bearing which comes from that assumption."

Fallow's own assurance was reflected two years ago when important people were beginning to notice him, assignments were coming from major magazines like Esquite and Atlantic Monthly. Fallows decided to quit Washington.

His wife Deborah , whose field is linguistics, was shopping for a graduate school and, when she chose the University of Texas, Fallows followed her to Austin where he wrote for Texas Monthly. Her course work is finished now and she is back in Washington, expecting their first baby within the month. She plans to finish her dissertation during the next year, the pursue her own career.

Charles Peters, editor of The Washington Monthly and admiring mentors, notes: "Jim had a willingness to get off the Washington ladder. One of the things that messes up most young men today is they're scared to get off the lader because they're afraid no one will let them back on."

Fallows, as a journalist, has been preoccupied by the "ladder" - the selecting and sorting process by which some young Americans (like himself) are picked for the top rung, the upper-class finishing done by universities like Harvard, the self-protective biases of the ruling elite or the meritocracy of whatever you wish to call it. Feeling of Guilt

PERHAPS HIS MOST compelling article in The Washington Monthly was a confession of his own complicity in this elitism; Harvard kids, himself included, were espousing antiwar moralisms and faking disabilities to dodge the draft, while the young men from blue-collar homes were drafted without protest and sent off to do the fighting, to be killed or wounded.

At his own draft physical, Fallows faked psychiatric problems and starved himself down to 120 pounds (he's now back to a thin but healthy 155). It worked but Fallows still feels the guilt.

"I was overcome by a wave of relief which for the first time revealed to me how great my terror had been," Fallows wrote, "and by the beginning of the sense of shame that remains with me to this day."

As Fallows points out, even this self-analysis can be seen as an expression of elitist arrogance - a luxury of confession available only to these are supremely confident of their place on the ladder.

"There is a little of the tone of the young girl in the white pinafore handing out soup at the soup kitchen," he observed. "You're privileged to write about these unjust class divisions when you feel yourself that you've come out on top of them."

In tha regard, according to colleagues in the Carter organization, Fallows has performed privately as one who raises dissenting questions inside the inner councils. During the campaign, Fallow wrote a memorandum warning against the gradual process of closing off access to Carter or shunting outside suggestions.When the memo turned up in the press, characterized as "creeping Haldemanism," Fallows got reprimanded.

The last article Fallows wrote for The Texas Monthly awarded candidate Carter the "Exceptional Piety Award" for some of the campaign bilge.

His summary opinion now of the White House staff: "If there are any monsters here, I don't know of them. There are people who are very cunning. There are people who are ruthless. But most of them have at some point what I would call a conscience or a sense of right and wrong."

A White House aide observed wryly of Fallows:

"There are few of us who won't be corrupted, but Jim will be corrupted less than most."

In addition to these other virtues, Fallows is a nice person. This is what everyone says about him. Nice does not always survive in these realms; detachment often gets plowed under in politics.

Still, Fallows does not feel threatened. He has his own well-defined turf and is not interested in poaching. The White House infighting is less intense now than he anticipated, certainly less hectic than what he saw during the campaign. "I may be naive, but I don't feel that I'm part of some sort of Darwinian struggle for life," he said.

Maybe so. Meanwhile, those bureaucratic predators who are tempted to step on this bright young man who is also nice and candid might keep in mind what one of Fallow's close associates pointed out about Jim: "He's a writer and, when writers go back to the typewriter, they're not as nice as they are in person."