The driving force behind the fictional "documentary" entitled "Interview with the Assassin" is an intimidating, portentous, volatile, paranoid performance by low-profile character actor Raymond J. Barry ("Training Day," "The Deep End"), playing a grayed, acrimonious ex-Marine sniper who claims -- now that he's dying of cancer and has nothing to lose -- to have been the elusive, legendary second gunman on the "grassy knoll," the real killer of President John F. Kennedy.

Barry induces goosebumps the moment he sits down in his sparse, TV-tray-decorated living room, in front of the digital camcorder of his nervous neighbor, an unemployed TV cameraman who was enticed by the man's quizzically vague promise to reveal a big secret that would make an even bigger story. "I don't wanna talk to the police about it. I don't want to go to jail. You don't show this to anyone until I say so," he fumes with preemptive menace. "Got it?"

The confession that follows is so disturbingly matter-of-fact that the man cannot be summarily dismissed as a crackpot. Finding out if his story is true becomes an obsession for the cameraman (Dylan Haggerty), and for those of us glued to the "Blair Witch"-style intimacy, immediacy and tension of the "raw" video footage that makes up the film.

Haggerty's camera follows Barry to a safe deposit box where he reveals a bullet casing that proves to be from the right era and the right caliber weapon. Then they travel to Dallas, where seeing Barry in Dealey Plaza causes an involuntary shudder. As he describes his timetable from November 22, 1963 (39 years to the day of this movie's release), he pantomimes a rifle in his hands while standing behind the fence where conspiracy theorists say the mystery shooter must have been that fateful morning. And he's so cold, callous and collected it's hard not to believe him.

It isn't long before both men are being followed -- or is it just contagious paranoia? -- while searching the country for the former Marine commander that Barry claims hired him for the hit. And the whole time, Barry is slowly seeming more and more dangerous. You get the feeling he could kill his interviewer if the mood strikes him, especially after he persuades Haggerty to buy a rifle and a handgun on his behalf, while wearing a hidden camera.

"You're gonna buy it," Barry says threateningly, citing a law that prevents convicted felons from acquiring legal weapons. "Ever heard of computers? I can't buy a gun."

Then he sneaks them onto a plane, just to show off.

Writer-director Neil Burger builds almost seat-gripping stress over the film's 88 minutes, as Haggerty's skepticism leads him to interview Barry's ex-wife on the sly and his fear leads him to install security monitors at his home, where his own wife and daughter have become panicked about their safety. And all the while, the handheld cinematography serves to make you feel you're in just as much danger as if you were looking through the camera's eyepiece yourself.

"Interview" stumbles a bit in the wake of its alarming climax, if only because there's a temptation to start second-guessing Haggerty. He ends up in what seems like an avoidable position. But the film does, from beginning to end, keep in line with the unique and chilling tradition of great Kennedy conspiracy lore.