JUST HOW PRESIDENT Reagan performs his job remains something of a mystery. We know that he takes it easy, often reaching the office at 9 a.m. and leaving at 5 to work out in the gym -- earlier when he goes horseback riding. We know he likes one- page "minimemos" and that he delegates a good deal of authority. We've often heard how he answers difficult specific questions with anecdotes of dubious relevance.

But what does he do as president? Does he meet with people to find out things, or does he only get briefed when his duties require it? Does he engage in serious give-and-take with staff or outsiders? Does he try personally to figure out issues? Is there more to Reagan than we realize?

Not much, judging by conversations with nearly two dozen people who have met with President Reagan sometime this month. Their accounts suggest a man who prefers fun to work, who loves to entertain and feel loved, who likes being president, but who rarely gets enthused about governing. They indicate that the president can get his details right, but is most likely to do so on matters he finds pleasing or useful in pleasing others. Details of the great public issues of our time don't always hold his interest. This is a president with an effective mechanism for avoiding what he doesn't like.

On Feb. 2 the president met in the Oval Office with four Jewish leaders, his national security adviser and others to discuss the Mideast. Reagan had invited the group to the White House as an alternative to his speaking to a gathering of the World Jewish Congress then taking place in Washington. The president evidently wanted to avoid being cross- examined publicly by these Jews.

Reagan asked his visitors what was on their minds. According to sources present, Julius Berman, chairman of the conference of presidents of major Jewish organizations, told Reagan that it wasn't fair to pressure the Israelis, who have always been willing to negotiate with any Arabs who would talk to them. It was the Arabs, he said, who put preconditions on their willingness to negotiate.

Reagan responded that he knew from his conversations with King Hussein of Jordan that as soon as an agreement on withdrawing foreign troops from Lebanon is reached, Hussein will be ready to negotiate with the Israelis. Berman reportedly pressed the president: Did that mean Hussein had put no other preconditions on his willingness to talk with Israel? Did he say nothing, for example, about Israeli settlements on the West Bank?

Before Reagan could reply, Judge William Clark, his national security adviser, jumped in with what struck his listeners as a vague answer. He suggested there were still other unresolved issues, such as how the Palestinians would be represented, though he never directly contradicted Reagan.

The president looked at his guests with an expression some interpreted as sheepish. You might think there is a little confusion here, he said, but there really isn't. Some of his conversations with King Hussein were private, just between the two of us, he explained; others involved his aides.

The brief meeting ended without any clearer explanation of Hussein's position. Reagan left the impression that he hadn't mastered the intricacies of the Mideast; Judge Clark's behavior suggested that Reagan's advisers feel they have to protect him.

That meeting was unusual for Reagan because it brought him together with outsiders whom he knew to be strongly critical of his policies. Though his aides have been trying to expose Reagan to more points of view, he rarely sees people who might challenge him directly. But this meeting was hardly dramatic, and it in no way advanced the president's knowledge of Jewish attitudes or the Mideast situation.

After that private meeting Reagan went to the East Room where about 150 Jewish leaders were waiting. He read a quick speech, then said his schedule compelled him to leave. He left two aides behind to answer questions. "It's like the campaign again," said one man who was present. "They don't want him to goof. . . He had been programmed."

That afternoon, the president had a brief meeting with six Afghan citizens who were in America to publicize a Soviet military operation which, they said, killed 105 civilians in their village. Reagan charmed the group, and convinced his guests that he was deeply concerned about their situation. According to Americans who accompanied the Afghans, Reagan proved himself an extraordinarily quick study, remembering key points in the stories of all six Afghans, so he could explain them to reporters who came into the meeting before it broke up.

The next day Reagan held a successful meeting with leaders of the major veterans organizations. According to Cooper Holt, executive director of the VFW, Reagan was warm and genial: "President Reagan is terrific with small groups." The subjects were national defense and the Veterans Administration budget. "He seemed to know exactly what we were talking about," Holt said.

One reason why it was a friendly meeting is that Reagan's new budget is generous to veterans' programs. It includes construction outlays about double the recent level and higher than any in 20 years, Holt said. Reagan knew the budget details, he added. The visitors expected a 15-minute session, but stayed 45.

Sen. Rudy Boschwitz (R-Minn.) and his son had a similar experience. They chatted with Reagan for 25 minutes, mostly about a speech Boschwitz had given commemorating the 50th anniversary of Hitler's rise to power, an event that prompted Boschwitz's father to leave Germany. "He engages you, he carries the conversation," Boschwitz said. Indeed, sometimes it's hard to get a word in. "What are you supposed to do? You can't interrupt, exactly, a president."

This president has a talent for making people feel good -- the city fathers of Monroe, La., for example. Reagan visited Monroe last month to inspect flood damage, then declared it a disaster area. On Feb. 7 a group of business leaders came to the White House to bring Reagan gifts and a 20-page scrapbook of letters from 6th graders in Monroe.

Rep. Jerry Huckaby (D-La.) was at the White House meeting. "Reagan took really more time than I thought he would," Huckaby recalled. The president told stories about how he had grown up in a river town and knew what floods could do. He carefully turned every page of the scrapbook he'd been given. "The people (from Monroe) were extremely awestruck," Huckaby said.

According to John Rea, president of the Monroe chamber of commerce, the time Reagan devoted to Monroe didn't end with that 22-minute meeting. Later all the 6th graders who'd written to Reagan got a photo of the president with a personal note written on it. And their teacher at Drew School got a handwritten letter from Reagan, explaining that he had tried to give each inscribed photo a personal touch so the kids would know it had really been written by their president.

That same morning the president had breakfast with nearly all the new Republican members of the House. Among those present was Rep. Ed Zschau (R-Cal.), a Silicon Valley entrepreneur who has turned to politics. Zschau sat at the same table with Reagan, and reported that it was a genial breakfast:

"We talked about the old days when he did baseball reporting. He told a few anecdotes. We didn't get into any heavy discussion. . . It was kind of the way he appears to the general public on television."

"He doesn't seem to probe people on their views, or solicit inputs as you'd think he might," Zschau went on. "When I was a company president, I never missed an opportunity to find out what my employes were thinking," but with Reagan "the conversation stayed on a rather superficial level . . . If I had been in his shoes, I'd have been more interested in finding out where those Republicans were coming from."

When Reagan met with his MX missile commission and its distinguished counselors -- a group that is supposed to find a plausible way to base the missile Reagan wants but Congress views warily -- he stayed for 15 minutes and made a few general remarks. The room was filled with former defense secretaries and high factotums of previous administrations, but one of them said later, "I had the feeling he didn't have any idea who some of us were."

When the president spent 10 minutes with Harold Willens, a California millionaire who backed the nuclear freeze referendum in California last fall, Willens perceived "a bit of almost stiffness on his part -- something a little more formal than others would have behaved in that situation." The meeting took place at the urging of Reagan's daughter, Patti Davis, herself a freeze advocate. "We were talking more past each other than to each other," Willens said, adding that Reagan was utterly gracious throughout.

When both Reagans went out to dinner at the home of Sen. Mark Hatfield (R-Ore.) with a group of presidential biographers and historians, the atmosphere was jovial. The historians suggested ways Reagan might help future scholars grasp what was really going on in his administration -- keep a journal of how you feel at important moments, for example. Reagan warmly agreed to this and every other suggestion the historians made. "He conveys kind of a 'gee whiz' feeling," one guest recounted. "Once he said, 'When I was a lifeguard at that pool seven days a week, I never thought I'd be president.' He's certainly not a terribly reflective man."

Perhaps the best example of Reagan at work that involves people other than his inner circle of advisers are his meetings with the Republican congressional leadership. The men in attendance watch Reagan closely, not least because several of them dream of sitting someday in his chair.

They seem struck by the fact that the president handles his role differently than any of them could imagine doing it. He seems removed from the details of most issues. He likes to structure the meetings with some care, and distinguishes between general meetings and specific sessions devoted to resolving a particular problem. The ordinary meetings usually proceed according to a fixed agenda; the president dominates them with his declarations on what he hopes can be done about various matters.

When the Republican leaders want to deliver a message, Reagan listens graciously, but rarely gets into a meaty discussion of the differences between them. He relies on aides to provide argumentation for his views. Sometimes his observations suggest that he isn't carefully following the discussion.

Reagan has the same difficulty with factual details in these meetings that he has sometimes had in public. "The details are killing him," according to one sympathetic member of this group. "Details are missing from his thinking process. That's a political negative for him -- though it's not necessarily non- presidential."

But more important than his grasp of detail is a side of Reagan that is visible to this group but which hasn't become part of the president's public image -- his ability to change his mind. "He's much more open than the image that's projected," one participant said. And when he agrees to change his mind -- by supporting a $100 billion tax increase last year, for instance -- he embraces the new policy with a bear hug, as though it had always been his idea. As far as the Republican congressional leaders are concerned, this is Reagan's most valuable quality.

A famous Hollywood actor -- though not a close friend of the president's -- dropped by the White House with his wife one afternoon around 4 p.m. just to say hello to Reagan. The president kept them in the Oval Office for an hour, then took them up to his gym. Finally, at 6:30, the actor insisted he had to leave. Reagan, who was loving the conversation about Hollywood, wanted them to stay for dinner. During the entire 21/2 hours, comithere was no interruption of their conversation by any of the president's aides.

"What you see is what you've got," observed Rep. Ed Zschau of Reagan. That seems about right. Is this bad -- or, more important, is it dangerous? Yes, maybe it is, particularly if his unbelievably good luck were to change. It seems amazing that after 25 months in office, Ronald Reagan has yet to deal with a real crisis. How would he handle serious trouble? Perhaps that is a question we shouldn't want answered.