Sixteen television cameras were on the South Lawn of the White House facing the diplomatic entrance. Two technicians with remote cameras tried to get a good angle from the White House steps. Nancy Reagan watched from the second-floor balcony.

Enter the president.

A few words for the cameras about the trip he was beginning to South America. Then, walking backward, he waved to the First Lady. With a look of melancholy she threw him a kiss. The president touched his lips and waved again while moving to the waiting helicopter. His jacket blew in the wind as he broke into a trot.

This is the nation's stage. Front page. Lead story on the evening news. Bulletins by the hour on radio.

The president looks older in person. It is not so much the slight graying at the temples since he took office, but rather the sagging, creased flesh under his chin and the way he nods his head as he talks. But this is not to say Reagan looks 72, which he will be Feb. 6. He moves vigorously.

And he dresses with flair. Leaving for Camp David one Friday afternoon, he wore a cream-colored suede jacket with a white fur collar, a white turtleneck and brandy-colored cowboy boots buffed to a high tone. By comparison, Mrs. Reagan looked dowdy in a down-filled coat as they walked together to the helicopter.

The president loped along, shouting to reporters, "No decisions," as they called out questions about the budget. One reporter shouted, "What about the unemployed?" but the president turned away. Seeing a Marine by the helicopter, the commander-in-chief snapped off a smart salute.

As the Reagans reached the top of the steps to the helicopter, the president held his hand high as if to wave goodbye. Mrs. Reagan, who had already ducked into the helicopter, came back out to wave, too. Picture-perfect for the weekend news.

Reagan's trademarks in White House ceremonies and media appearances are charm and an aversion to details. Appearing suddenly in the White House press room with a group of congressmen in December to announce what the White House claimed was a bipartisan compromise on the MX missile, Reagan was laughing with the others coming into the briefing room.

"Why are all these folks smiling?" a reporter asked. After some mumbling, the president, still smiling, said, "No, it's really just that we're overjoyed at the opportunity of meeting with all of you."

After Reagan talked for several minutes about an agreement having been reached, a reporter raised a hand to ask what the agreement was. "The compromise is going to involve," the president began and then cut himself off: "Would you like to explain what the compromise is, John Tower?"

The conservative Republican senator from Texas laid out the plan, and Reagan talked some more about the importance of an agreement before saying, "But now I am going to turn you over to these gentlemen who can answer your specific questions on the subject of the day."

"You don't tell us how to stage the news and we don't tell you how to cover it," says a sign on the desk of White House spokesman Larry Speakes.

At a briefing a few weeks ago to counter what he called "gloom-and-doom" reporting, Speakes released government budget projections showing economic growth of 3 percent from December, 1982, to December, 1983, although, as had been reported, the gross national product for all of 1983 was projected to rise only 1.4 percent above the 1982 GNP.

"I've given you the lead," Speakes said. "Go with it."

When a reporter asked if Speakes also would release projections for unemployment or news of defense budget cuts, Speakes said no. He was releasing the GNP numbers only because of the "gloom-and-doom reporting."

"Does this mean," asked Bruce Drake of The New York Daily News, "that if we write doom-and-gloom stories about the defense budget you'll release information on that, too?"

Because there is little real information from the press office on what goes on inside the White House, reporters ask every day: What did the president mean when he said . . . ? Was that leak true? What did the president do today? Any decisions? Did he accept any resignations?

That last question was asked at one of Speakes' noon briefings in mid-December by ABC-TV news correspondent Sam Donaldson, who had a tip that Transporation Secretary Drew Lewis had resigned. Speakes said there were no resignations.

"So I have your word that there will not be a letter of resignation dated Dec. 13 before 1:10 p.m. from any Cabinet officer?" Donaldson asked. Speakes said yes.

When it was announced weeks later that Lewis had resigned in a letter sent to the president Dec. 13, Donaldson asked Speakes why he hadn't told the truth.

"I said I wasn't aware of it and I said I'd check," Speakes responded. "I didn't check."

The White House briefing room, with seats for nearly 60, is filled for the noon briefing every day with reporters from the wire services, big-city newspapers, network radio and television and foreign news organizations. The briefings are sometimes unruly, and the questions and responses rude.

When reporters were allowed into the Oval Office recently for an impromptu news conference with the president, Speakes warned, "You screw it up and it doesn't happen anymore. It's as simple as that, I can guarantee you that."

At a later Oval Office news conference, a White House press aide went around the room asking newspaper and radio reporters to move back so television reporters could occupy positions in front of the president's desk. Only the television reporters got to ask questions in a session carried live on several networks.

Television and radio depend on the press office for advance notification of any "photo opportunity" to film and record the president, for which camera and sound technicians need time to set up bulky electronic equipment. Dozens of dungaree-clad technicans lounge around the White House all day, every day, ready for any presidential appearance.

Reporters also rely on a daily flow of White House news releases and twice-a-day briefings by Speakes, who brings with him a sheaf of papers. If a reporter asks the right question, out comes a sheet of paper with a statement. Usually there is no elaboration beyond that.

On the day after it was reported that Health and Human Services Secretary Richard S. Schweiker was resigning and was likely to be replaced by former representative Margaret M. Heckler (R-Mass.), the president was scheduled to make an announcement at 10 a.m..

At the 9:15 a.m. briefing in Speakes' office, frenzied radio and television reporters blitzed him with questions: Is it Heckler? Is it a female? Is it a person from the Northeast?

"If you'll wait 41 minutes, you'll find out," said Speakes.

"Covering the White House," said one reporter who recently left the beat, "is like being trapped in the locker room at a football game. You hear the crowd, you know something is going on. When they let you up to see what is going on, you see a lot of activity, but later you find out that was only the half-time show."

A junior White House aide recently said of his job: "To work in the White House is to feel like you are at the center of the world. You know what is really going on in the world."

But even most aides are outside "The Loop," which is what staff and secretaries call the people in touch daily with White House chief of staff James A. Baker III, presidential counselor Edwin Meese III, deputy chief of staff Michael K. Deaver or national security affairs adviser William P. Clark. The limited focus on who has power bruises egos among aides outside The Loop.

Sometimes even the president appears not to be part of The Loop until he is briefed fully on what to say or do. Reporters joke that Reagan speaks in tape-recorded cassettes plugged into his head by The Loop: Social Security is a "political football," government programs for the unemployed are "make-work" jobs programs.

Gossip in the White House is mostly about rivalry between Baker aides and Meese aides, about who was left out of what deal, or about who leaked such-and-such a story to the press for what reason and on whose authority. Sex is not a hot topic. Secretaries laugh when asked if there are White House affairs.

Every day, television cameras wait outside the West Wing entrance of the White House to record visitors' impressions.

"Well, Mr. Biden, what do you think the president will do with the budget?" a reporter asked Sen. Joseph R. Biden Jr. (D-Del.) after he had been in to talk about a crime bill Reagan later vetoed because of an amendment added by Biden. "People who shrug their shoulders at 10 percent unemployment don't know how revolutions start," Biden said.

What about the deficit, Senator?

"The train is leaving the Hill and coming down Pennsylvania Avenue and it will take the White House with it," Biden said in forecasting demands for reducing the deficit.

As the clump of reporters around him began to break up, one shouted out: "One last thing, Sen. Biden, could you give us a colorful quote?"