An actor's performance as a mentally-challenged character can come off as a stunt designed for Oscar attention more than a role in service of a compelling story, and Sean Penn in I Am Sam skirts dangerously close to fostering this perception. However, Penn's performance is both credible, for the most part, and has some shading (thank God-- he's in nearly every scene of this two-hour-and-twelve-minute heart-string concert).
Penn plays Sam Dawson, father to seven-year-old Lucy Diamond Dawson (Dakota Fanning). When a suspicious social worker conveniently catches Sam and Lucy in a rare moment of domestic disharmony, Lucy is unceremoniously yanked from her home. Michelle Pfeiffer plays the selfish lawyer reluctantly stuck defending Sam (a frustrating client) for free. Let's just say she has a change of heart (though the filmmakers wisely decided against an overt romantic subplot between Pfeiffer and Penn).
The film earns substantial good will from its lead performances. Complimenting Penn (perhaps the best actor of his generation) is the excellent Michelle Pfeiffer. Unfortunately, her character is strictly stock, the selfish, upwardly mobile businesswoman who needs to be taught a lesson in what really matters (hint: it ain't money). The biggest surprise is Fanning. An ultracute kid who one might assume comes direct from central casting, Fanning displays a disarming directness (with a gaze that is equally comfortable conveying emotion and laser-like focus), without which the movie would surely be lost.
Penn, though generally good, suffers from the company of a group of actors playing his challenged friends (the usually brilliant Doug Hutchinson, for example, overplays it here, throwing more light on Penn's dangerously showy performance). Worse, Penn must act alongside real mentally-challenged actors, at times, and the comparison again detracts from Penn's mannered work.
The biggest problem with the film is its predictable frame and familiar characters. The story offers a minimum of ideas, though its central one is notable: what happens once the child is demonstrably smarter than the parent, especially if the child is still a grade-schooler? The film's answer reveals the film's unfortunate tendency toward plot contrivances. Aside from the all-star cast, the film plays all-too-much like an old-fashioned TV-movie-of-the-week, with it's predictable rise and fall of fortunes, conflict (court provides the crutch) and tidily resolved ending.
But I Am Sam ultimately provides that concert of the heart, craftily orchestrated by director Jessie Nelson. Nelson's successful pursuit of Beatles songs (covered by a variety of popular artists including Eddie Vedder, Sarah McLachlan, and Rufus Wainright) adds a layer of irresistable sentiment and commentary to a story that repeatedly reminds that all one needs is love.