When it comes to making Camp David helicopter departures, Nancy Reagan is normally a study in well-dressed composure. Whatever she may think of questions being hurled at her husband by reporters, her face rarely betrays emotion or acknowledges involvement in White House affairs.

But Mrs. Reagan, who leaves the issues to others, can become very deeply involved as a judge of Reagan's subordinates when she perceives that her husband's welfare is at stake. For a brief moment at Reagan's departure July 8, the First Lady allowed a rare on-camera glimpse of the role those close to her think she is likely to play in the controversy surrounding the Carter debate briefing documents.

The scene was a carefully scripted three-minute morality play on the South Lawn of the White House, in which Reagan was cast as The Leader who orders his scattered minions to cooperate with Justice Department investigators in pursuit of truth. It was good theater, complete with Reagan's casual revelation that he had never heard about the effort of his 1980 campaign to use retired military officers to monitor an anticipated "October surprise" by the Carter camp.

But more important than what the president said was what Mrs. Reagan did. As the engines of the helicopter Marine One were started, a television reporter shouted at the departing president, "Will you fire people if you have to?"

Reagan ducked the question, saying he would take whatever action was necessary. The reporter asked again whether this included firings. The president turned toward the cameras, with his wife close beside him. He said, "Yes." She said nothing but nodded her head vigorously several times while looking directly into the television cameras.

Those who saw this, or were told about it, remembered other times when Mrs. Reagan thought it important to get involved. Some recalled that her involvement had strongly influenced the present careers of the principal antagonists in the briefing-book drama, CIA Director William J. Casey and White House chief of staff James A. Baker III.

It was Nancy Reagan who perceived the personal damage being done to her husband in the early months of 1980 by an emotionally draining power struggle for campaign leadership. She played a key role in pushing out John P. Sears as campaign manager and replacing him with Casey.

And it was Nancy Reagan who in the hours of her husband's victory agreed with Michael K. Deaver and campaign strategist Stuart K. Spencer that Baker, not longtime Reagan adviser Edwin Meese III, was ideally suited to become White House chief of staff. Baker still has that job and, according to all reports, Mrs. Reagan's approval.

In marked contrast to her husband, a friend of both said, Mrs. Reagan is capable of taking a "pragmatic, hardheaded view of what is needed to be done when it's time to make a change."

"She is," the friend added in deliberate understatement, "somewhat less sentimental than her husband."

The debate-book drama is not going to be finished in a few minutes or even a few days, and the ending has not been scripted as carefully as Reagan's goings and comings to Camp David. But those inside the White House increasingly are saying that a probable climax will involve ouster of whoever on the Reagan campaign side was responsible for obtaining the materials, if that "someone" is still in government.

And when that time comes, if it does, Mrs. Reagan may have more than a bit part to play.

The White House is worried about "slippage" among House Democrats on the upcoming MX authorization vote but still expects to prevail. Recent movement, or appearance of movement in the strategic arms talks at Geneva, has probably helped.

The Scowcroft MX commission continues to shine as one of this administration's most effective political accomplishments, whatever one thinks of MX. At a dinner with the commission last Thursday, President Carter's defense secretary, Harold Brown, delivered what was described as an "eloquent" toast to Reagan.

This column traced the Reaganesque phrase "feet people," used to describe Latin American refugees, back to a Conservative Digest news release issued by Richard A. Viguerie April 28. Now comes Dana Rohrabacher, the conservative White House speech-writer who put the words in Reagan's speech, with an earlier origin.

According to a clipping in the Jacksonville (Fla.) Journal on March 1, 1982, J. William Middendorf II, U.S. ambassador to the Organization of American States, used the phrase in a speech to eight Rotary Clubs the previous day. In words almost identical to those now used by the president, Middendorf said: "We will have not just boat people. We will have feet people."

When Reagan refers to feet people he is talking about prospective refugees who he says will flee Marxist takeovers in Central America. The "hordes" anticipated by Middendorf are also refugees from economic conditions, according to the news story.

Reaganism of the Week: (To Secretary of State George P. Shultz during a photo session in the Oval Office last Wednesday): "Isn't it amazing? You might have 50 ties in your closet, but then you only wear half a dozen of them."