Every so often, you can't help but judge a movie performance by its cover. That is, you see quickie excerpts of it in a TV commercial and absolutely know you'll hate it. And, based on CBS's ads for ''Riding the Bus With My Sister," it's easy to think Rosie O'Donnell will be intolerably sappy as a mentally challenged woman obsessed with public transit. In the ads, the former daytime host looks like she's on a raging mission to pluck your heartstrings until they hurt. The images make you wonder if, after all, Sean Penn was actually underacting in ''I Am Sam."
But O'Donnell is far more successful in the role than you might expect. She brings great honesty and intuition to this otherwise thoroughly bogus Hallmark Hall of Fame production, which premieres tomorrow at 9 p.m. on Channel 4. Her Beth is not just another one of entertainment's condescending takes on the mentally challenged, a wounded bird too cute and pure for the cynical world of adults. She's an angry and loud woman who can deploy hurtful comments to friends and strangers alike. While O'Donnell does employ some of the clichéd hyper-innocence we saw in ''Forrest Gump" and ''Sling Blade," she also brings along plenty of unsympathetic sourness. Her childlike energy has bite.
The role seems to provide O'Donnell with an opportunity to use her own personal tumult, the moodiness that has emerged in her off-screen personality since she left her talk show in 2002. Long pigeonholed as the Queen of Nice, she has morphed into a figure of bitterness, activism, and pride, freely venting her venom in passages on her blog. All of that disquiet and abandon seems to find an outlet in ''Riding the Bus With My Sister," as Beth yells her way through her days, her facial expressions exaggerated, her arms forever grabbing. It takes a good 10 minutes to adjust to the fact that O'Donnell's lack of control is not meant for comic effect.
Alas, it's much harder to adjust to the rest of the movie, which is as predictable as can be. Directed by Anjelica Huston, ''Riding the Bus With My Sister" is also the story of Beth's stuck-up older sibling, Rachel, played stiffly by Andie MacDowell. A photographer who's unable to love, Rachel is forced to take care of her sister once their father dies, in the same way Tom Cruise was responsible for Dustin Hoffman in ''Rain Man." But of course Beth is doing quite well, thanks to an extended family of bus drivers and passengers, while Rachel is the one who needs to make serious life changes. It's not hard to know where this bus is going to stop, even if there are a few nice surprises along the way.
Matthew Gilbert can be reached at email@example.com.