President Reagan's top advisers, in negotiating the debates with Walter F. Mondale, made a point of insisting to Mondale's strategists that Reagan be allowed to use a specially equipped podium that will amplify questions so the president can hear them Sunday.

That demand offers a small glimpse into the instinct of Reagan's political team that the debate will be decided not only on how the candidates handle issues but also on their tone and "body language" before a national television audience.

In addition to a blizzard of briefing material and a debate rehearsal with top aides and Office of Management and Budget Director David A. Stockman, Reagan is planning to watch a 40-minute videotape of Mondale from several of the Democratic primary debates last spring.

Prepared by the Republican National Committee, the tape is designed to give Reagan a sense of how Mondale attacks and reacts under fire.

Aides said Reagan was preparing for a broad-based Mondale challenge on domestic and foreign issues. One administration official said Reagan strategists were expecting Mondale to "come out swinging" on the theory that the Democratic nominee has everything to gain and little to lose Sunday in the first of the two debates.

Specifically, they expect Mondale to raise the issue of the embassy annex bombing in Beirut Sept. 20 in which two Americans died, even though the first debate is supposed to be on domestic issues.

"Mondale's so far behind that he has to make something happen in this first debate to get back into the race," one official said.

Another Reagan aide said he thought that Mondale would benefit from the moment both candidates walk on stage because of the national exposure he receives as challenger.

"He wins by virtue of being on the screen with Ronald Reagan. It puts him on the same plane with the president," this official said. "To win, Reagan must obviously knock the guy out. For Mondale to win, all he has to do is stay in."

But this official suggested that "Mondale could win the debate and Reagan win the audience. So much more is important than content -- looks, tone, overall impressions."

In his many prepared campaign speeches and his 18-minute filmed convention advertisement, Reagan has tried to project confidence and optimism. Aides want him to project the same image in Louisville Sunday, since Mondale is certain to portray him as out of touch.

The Reagan team was concerned that a misunderstood question or misdirected answer could mar Reagan's style. Their most serious concern was that Reagan, 73, might do or say something that would raise doubts in voters' minds about whether he is up to the presidency.

The president wears a hearing aid because he has difficulty hearing in one ear, the result of a gun fired near him on a Hollywood movie set many years ago. He often responds to questions by asking, "What?"

Last week, when Soviet Foreign Minister Andrei A. Gromyko referred to the "hurricane" of photographers recording his historic meeting with Reagan in the Oval Office, the president misheard and started to tell him about the hurricane off the Florida coast.

Monday, before the Detroit Economic Club, Reagan could not hear two of three questions read to him at close range.

Thus, the president will use the special podium that is used during White House news conferences to amplify each question. A Mondale campaign official said debate negotiators struck a deal: Reagan could use the special podium and Mondale could have the session last 90 minutes.

That deal was struck by White House chief of staff James A. Baker III, who has ordered that Reagan's subordinates not talk about debate preparations.

Others helping to prepare Reagan include presidential assistant Richard G. Darman; campaign speech writer Ken Khachigian and Stockman, who impersonated John B. Anderson and Carter in 1980 debate rehearsals and will act as Mondale this time. Stuart K. Spencer, Reagan's political strategist, and deputy chief of staff Michael K. Deaver also are helping.

Aides said that Reagan was confident and remained convinced, as he has said, that the only debate he ever lost was one at which he did not appear -- in the 1980 Iowa Republican caucuses.

"With Reagan, you have to go with his instincts, which are what got him where he is," one official said. "Reagan is going to be Reagan."