WASHINGTON (AP) _ As the man who walked one step behind President Reagan for 31/2 years, Robert L. DeProspero began every day knowing he could be called upon to place his body between the president and a bullet.

There never was any doubt that he'd do it.

''I always felt I was the guy responsible,'' he said. ''There's no question that if you made a wrong move or did not do what you were supposed to do it would ruin your life, your family's life and would ruin anyone who was associated with you.

''That's what I felt. Those were the pressures I felt.''

DeProspero, 46, reflected on his just-concluded tour as special agent in charge of the presidential protective detail in a recent interview with The Associated Press. He recalled learning of the assassination of John F. Kennedy in 1963 while he was driving home from a high school where he was a teacher and coach. That was two years before he joined the Secret Service.

''I never did consider myself to be an emotional person, but I found myself with tears in my eyes,'' he said. ''I thought, what is the guy doing who's the head of the detail? He must be suffering as much as anyone else.''

And so when the protection baton was passed to him, that memory became a motivating force. ''I always think about what I did in that car driving home,'' he said.

Since he was recruited into the Secret Service in 1965, DeProspero has spent more time than any other agent protecting officials, including Dwight Eisenhower, Lyndon Johnson, Nelson Rockefeller, Edmund Muskie, Spiro Agnew, Gerald Ford, Jimmy Carter, and Ronald Reagan.

He especially remembers showing up unannounced at Rockefeller's home on June 7, 1968 - the day after Robert F. Kennedy was assassinated - to inform him that Secret Service protection had been ordered for all presidential contenders.

From that June until after the November election, DeProspero was on the road almost constantly. ''I was home probably five days,'' he said. ''It was difficult.''

His wife, Patricia, said she worried occasionally whether her husband's frequent absences would adversely affect their daughter, Robin, and son, Bobby. The kids never were encouraged to follow their father's footsteps, but Robin now works for the Secret Service - though she's not an agent - and Bobby, who is in college, recently announced that he's considering the Secret Service as a career.

DeProspero took charge of the presidential detail in July 1981 - just months after the attempt on Reagan's life - from Jerry Parr, the man who shoved the president into his limousine after the shooting began. DeProspero's successor is Raymond A. Shattick, who was at Parr's side that day.

In the aftermath of the assassination attempt and the rise of international terrorism, DeProspero presided over perhaps the greatest increase ever in White House security.

Yet in his mind, it could never be enough.

''I had a reputation for overkill and everybody thought I was overprotective, '' he volunteered. ''I knew I had that reputation and that didn't bother me.''

His method, simply, was never to let up - not on himself, not on a White House staff with whom there were inevitable disagreements about security- related matters, and not on the agents.

Before every presidential trip, DeProspero made it a habit to talk by telephone to the advance agent on the scene. Once, before Reagan visited Japan, DeProspero spent all night on the telephone resolving last-minute problems and then boarded on Air Force One for the 18-hour flight.

Between trips, DeProspero tried to inspire agents to remain at ''a high pitch of readiness,'' with occasional meetings and fitness competitions. The Secret Service refuses to say how many agents are on the presidential detail, but it's probably about 100.

The object was to help them guard against boredom in a profession that depends so totally on constant vigilance.

During the last weeks of the 1984 campaign, when a meeting was impractical because everyone was on the road, DeProspero tried to pep up his troops with a tape recording in which he referred to the years he coached his son in wrestling.

It always was the same theme: Never let up. ''You can be ahead 22-0, and if you ever let up the other guy can pin you and you've lost the whole thing,'' he warned.

DeProspero's single-minded pursuit of his mission earned him respect and sometimes fear, but not overwhelming popularity.

''I am not overly intelligent,'' he replied when asked if he had left a legacy. ''I am certainly not the most suave person. I don't have the greatest personality. But I think I have the ability to lead men and that's the place you can do it because you have a lot of good men and they are after strong leaders.''

DeProspero preferred to lead by example with a style that was a bit more formal than his predecessors.

For example, he began calling his deputies ''Mr.'' in front of the agents. They got the message and soon referred to him as ''Mr. D'' rather than ''Bobby,'' which is how he was known to the White House staff.

And his passion for physical fitness, considered a bit odd years ago when he would tote his weight-lifting equipment on presidential trips, quickly spread through the ranks when he took charge.

DeProspero runs for endurance, but lifts weights for strength because he believes that ''if I was called upon on the job to do something'' it probably would require more strength than endurance.

At his urging, a new gym was installed for the agents in the Old Executive Office Building across a driveway from the White House.

Now, when the president is on the road, it is much more common to find the off-duty agents running outside their hotel or lifting weights than sipping beer in the bar.

''I would say there was a lot more fraternization and friendship between the staff and the Secret Service in previous administrations,'' said William Henkel, director of advance at the White House.

He described DeProspero as a ''consummate professional'' who brought an ''icy calm'' to ''one of the more awesome jobs anyone could have.''

To DeProspero, the concept of overprotection is foreign.

''My philosophy was to do as much as the restraining groups (White House staff) would allow me to do,'' he said. ''I would do as much to shelter the president.''

While the White House staff traditionally has been a restraining influence on the Secret Service, Reagan's staff was much more sympathetic - even before the assassination attempt. As a result, Reagan became the least accessible, most protected of modern presidents until the 1984 campaign, when things loosened up greatly.

''What drives the agent in charge of the detail is: I don't want anything to happen to the president,'' he said. ''That's what drives you the whole time. That's my success.''