The problem with being a movie star is that fame, as a byproduct of success, tends to create boundaries for future success. Acting involves creating illusions, a process often counteracted by the familiarity brought on by celebrity. One of the better actors around, Sean Penn provides a perfect example of this paradox in I Am Sam. Playing a retarded coffee-shop employee fixated on The Beatles, Penn delivers a performance at once controlled, impressively detailed, and impossible to believe. Coming as it does from such a familiar face, it can't help but seem like something of a routine. If this were the least of I Am Sam's problems, it might not matter so much. Sadly, the film strands Penn in the middle of a story so manipulative that the appearance of mustache-twisting baddies never seems out of the question. As a single father, abandoned by the homeless woman who gave birth to his child, Penn accumulates lessons in parenthood through the help of his kindly agoraphobic neighbor (Dianne Wiest) and a Greek chorus of the fellow developmentally disabled. When the child grows into a wise-beyond-her-years 7-year-old (Dakota Fanning) and begins to surpass Penn's mental abilities, the arrangement develops problems, her friends' cruel mockery of Penn not the least of them. After a birthday party unexpectedly attended by a Children's Services representative goes predictably awry, Sam brings out the first of several scenes in which unsmiling authority figures cart Fanning away from her upset dad. Eventually, Penn guilt-trips high-powered, jellybean-munching attorney Michelle Pfeiffer into taking his case pro bono, and the film switches gears, becoming a tedious courtroom drama at least partially dependent on a phobia-challenging appearance by Wiest. Paced leisurely and directed with all the subtlety that might be expected of Jessie Nelson (a screenwriter on The Story Of Us and Stepmom), I Am Sam gives itself plenty of time to dilute its sap, particularly in a middle section that feels poised to deal with the actual issue of whether Penn is a good father. Other instincts eventually win out, however, as Nelson surrenders to the impulse to make Penn a lovable, misunderstood creature, not unlike the friendly yeti of Harry And The Hendersons. This is C-grade material that somehow attracted a grade-A cast. Perhaps Penn and company saw the film as the cinematic equivalent of working pro bono, but the final results make it hard to see who benefits.