"Don Carrington" is an earnest man in his early forties. He has written a screenplay about the life of a prominent member of Congress and has tried dozens of times to deliver it to her personally.

"I'll do anything it takes," Carrington tells Capitol Police officers Jennifer Meerman and Marc Miles. "If she gets this manuscript, she's going to love it! She's going to love me!"

"Carrington" is actually David Swink, a specialist in a peculiarly Washington discipline: handling the mentally ill and their threats to public figures.

He is playing a role in a recent training session for 12 men and five women from the Capitol Police, the 1,300-member force responsible for the physical safety of Congress's 535 lawmakers.

Alternately argumentative and informative, "Carrington" talks for an hour to Meerman and Miles about the congresswoman with whom he is obsessed. It is a splendid, chilling performance, ending when Miles asks: "What's your next move?"

"Well," Carrington" replies. "I'm certainly not going to tell you."

The Capitol Police started a new round of training sessions last year after Russell Eugene Weston Jr. broke into the Capitol and killed two police officers who, he said, were opposing his efforts to protect the United States from disease and cannibalism.

The training sessions are run by psychotherapist Barry Spodak of the D.C. Commission on Mental Health. Along with Swink, has worked for more than 25 years with law enforcement organizations ranging from the D.C. police to the Secret Service.

"We stay in the role," Spodak said. "And although we have a clear idea of what kind of person we're playing, how we react depends on what they [the police] do."

In the first simulation Spodak plays an aging hippie, wandering near the Capitol talking abstractedly about how "they're going to kill him." He tells police that "President John F. Kennedy" sent him to foil a conspiracy involving a "troika" of prominent Republicans out to "get" President Clinton.

Officers Yvonne M. Dove and Michael Spriggs expertly remove him from the public eye to where they can interrogate him. They are understanding, without being friendly, and they seem to know the drill by heart: "This is too much like everyday work," Dove says at the session's end.

Indeed, David Wells, the Capitol Police's supervisory special agent for threat assessment, says the force investigates about one threat per day by telephone, e-mail, regular mail or in person. And activities on the Hill can attract unwanted visits from the mentally ill.

Some incidents, such as overt death threats from paranoid-schizophrenics, can result in arrests and hearings. Others can be solved more simply, like the case of "John Milne," a nonstop talker also played by Spodak.

He is in the manic phase of bipolar disorder, and wants to join a senator's presidential campaign to offer a communications plan that will put the senator over the top. He has left his job "to dedicate myself to a higher purpose."

The hard part with "Milne," Swink says, "is to take control of the interview," and to have the patience to wait for opportunities to extract information from him.

Patience, however, will reward the interrogator, he adds, because "there's a high probability that if you get him back on medication, you're not going to see him again."

That won't be the case with "Carrington," the screenwriter, who is neither delusional nor manic and may, Spodak said, not even have a serious mental illness. "But he has an obsessional quality toward a public figure," Spodak said, a phenomenon that has become more widespread "as the border between celebrity and politics has blurred."

And after an hour with "Carrington," the Capitol Police are nowhere: "He's intelligent, sophisticated, and is a lot more dangerous than a lot of stalkers," Swink says at the end of the session.

More important, he didn't threaten anyone. So even though "you can send out a bulletin" on him, Swink said, "you're going to have to free him."

CAPTION: Capitol Police Officers Yvonne Dove and Michael Spriggs confront a deranged man, played by psychotherapist Barry Spodak in a role-playing exercise that teaches officers how to deal with mentally ill visitors.