Every once in a while a small film comes along that’s based on an idea so clever, and treats it so smartly, that the result pretty much disarms criticism. That’s the case with Neil Burger’s “Interview with the Assassin,” a wittily constructed and pleasurably unsettling pseudo-documentary that draws on the still-widespread belief that the murder of President John F. Kennedy was a conspiracy rather than the act of a single killer.

The set-up is wickedly simple: Ron Kobeleski (Dylan Haggerty), an out-of-work California TV cameraman, is approached by Walter Ohlinger (Raymond J. Barry), a gruff, grey-haired neighbor, to film his confession to a long-ago crime. Ohlinger claims to have been the rumored second gunman in the Kennedy assassination–the mysterious figure who supposedly fired the fatal shot from the infamous grassy knoll and escaped, leaving the patsy Oswald to take the rap. Walter, who also claims to be terminally ill, explains to Kobeleski that he’d been recruited for the deed by a superior under whom he’d served in the Marines, and that powerful forces are still at work to insure that the truth remains buried. The cameraman is initially incredulous but desperate for a story that could make his career, so he encourages Ohlinger to prove his assertions on film. Before long the duo is on the move across the country to locate people who can corroborate Walter’s unlikely tale.

A number of elements explain why “Interview” works as well as it does. One is the expertly gauged, brilliantly restrained performance of Barry; he captures perfectly the Walter’s world-weary matter-of-factness, his underlying menace, and his periodic glimmers of madness. Barry is good enough to keep viewers guessing about whether he’s what he says he is, or simply nuts; it’s one of those exceptional turns by a splendid character actor that won’t receive the acclaim it deserves, simply because it’s so expertly understated. (There’s an absolutely chilling moment when Ohlinger explains why he’d agreed to kill the president in the first place–for the purely banal reason of feeling powerful–and Barry captures it dead-on.) But Barry wouldn’t be so impressive if Burger hadn’t been so canny in constructing revealing episodes for the character. A visit to Dallas, where Ohlinger walks through the route he took on the day of the assassination, is eerily straightforward, and another, in which Ron and Walter visit one of the older man’s former comrades-in-arm for some target practice after buying a few guns, is right-on, too. The script ratchets up the tension by cannily inserting a suggestion that the duo is being followed–which in turn leads to some frighteningly extreme reactions on Ohlinger’s part. There’s also an interview Kobeleski conducts with Walter’s ex-wife (the pitch-perfect Kate Williamson) that’s beautifully shaped and played. And on a more general level, the sense of co-dependency that the film builds between the two men is subtly and effectively rendered: each is using the other for very selfish reasons, and both prove to be villains, though in different ways. The gritty, faux-verit√© style contributes to things as well; with the frequent POV shots from the perspective of Ron’s camera, the picture comes to resemble a more cerebral, though equally mysterious, variant of “The Blair Witch Project.”

But there are drawbacks, particularly toward the close. Simply put, the conceit, while clever, proves difficult to sustain to feature length, and the attempt to provide the shaggy-dog story with a hugely surprising denouement misses the mark. From the point when Walter and Ron reach Washington in their peregrinations, the plot twists grow increasingly strained, and by the final wrap-up, the earlier air of perverse plausibility has dissipated.

Nonetheless, for most of the distance the picture provides a satisfyingly unsettling ride into the dark places of our national psyche. We don’t get many paranoid thrillers like “The Conversation” or “The Parallax View” (or even “Three Days of the Condor” or “All the President’s Men”) anymore–the imperfect “Arlington Road” (1999) was probably the best recent example. In spite of its disappointing final fifteen minutes, “Interview with the Assassin” is a nifty little addition to the genre.