There is a wild card, though: It's Dean's performance. Ever since he had the misfortune to star in ''Billy Bathgate'' (1991), this actor hasn't been seen much on screen, but now, all grown up, he turns out to have a puckish slyness. Mumford is a born listener who manipulates people by treating them with respect. He's a contradiction -- a humane con artist. His remarks, even when reassuring, are verbal curveballs. He keeps his patients in a state of surprise by lowering his shrink's armor when they least expect it, and that's what Dean's timing, a kind of hyper-placid Zen vaudeville, does for the audience.
Mumford's words pop out faster than you expect, as if he were forever beating you to the pause to see what you'll say. The charm of the movie is in how far he'll go to get through to people. He trashes the patient-doctor confidentiality pact at a moment's notice, not to be nasty but to dramatize the fact that we all have hidden lives. When it turns out that Mumford himself isn't who he appears to be, it's a shock all right, but it doesn't violate him; it completes him. In a sense, he's the ultimate shrink: an imposter playing an imposter.
As characters, Mumford's patients, who are all tangled up in overactive fantasy lives, have a touch of TV-ensemble-comedy cuteness, but the cast helps to fill them out. Jason Lee is gawky and sweet as a local software magnate with a techno-geek's dirty secret, and Mary McDonnell hits notes of operatic silliness as a housewife addicted to boutique-catalog shopping. Pruitt Taylor Vince, as a sad-sack pharmacist who can't even bring himself to star in his own film-noir sexual fantasies, has a spiky verve.
The conflicts are too neatly resolved, but there's more at stake when Hope Davis, as a woman suffering from chronic fatigue syndrome, is on screen. Her sessions with Mumford, which consist of walking around with him as he falls in love with her, are melancholy and screwball at the same time.
Beneath its merry comic veneer, ''Mumford'' is really an arresting puzzle: Who is Mumford, and will he get away with being the most deceptive and unethical shrink this side of Hannibal Lecter? The film views therapists as ghost doctors who essentially work by de-stigmatizing people's secrets. It turns the tricks of psychology into duplicitous high play.