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One of the hardest things to pull off in a book-to-film adaptation is the characters' interior lives. A sufficiently fussy author can devote 10 pages to setting up the motives and thought processes behind a character's simple action, but an actor may have mere seconds to communicate those complexities. It's almost better for all concerned when the source material doesn't dig far below its characters' surfaces, or give them motivations that can't be explained in a word or two: "She's mad." "He's jealous." "They're all sad."

Which should make Jennifer Weiner's slick bestseller In Her Shoes fairly ideal for a transition to the screen. The film version cuts out some of the steps, but it still follows the novel's story of two sisters—bubbly, pretty, dysfunctional spoiled brat Cameron Diaz and her only-by-Hollywood-standards plain, dumpy sister Toni Collette—who've envied and resented each other since their troubled childhood, but come to terms anyway when a particularly bad spat parts them, then enables both of them to get what they most want in life. The story is occasionally baffling in the gap between sensible reality—in which prim, fussy, judgmental lawyer Collette and thieving, irresponsible, selfish layabout Diaz would probably run screaming in opposite directions and never speak again—and happy Hollywood closure, which has them constantly pining for each other. But while their motivations aren't always realistic, they're simple and clearly stated.

The film version inherits some of its problems directly from Weiner's novel, particularly the trite relationships and Collette's suitor (Mark Feuerstein), the type of obsessive, insinuating control freak only valued in romantic comedies. But book-to-film wunderkind director Curtis Hanson (Wonder Boys, L.A. Confidential) comes by some of the trouble honestly on his own. His last film, 2002's 8 Mile, was gritty, propulsive, and heartfelt; In Her Shoes lacks any of that sense of investment. Scene after scene emerges with little sense of connection, direction, or momentum, and by the time Diaz flees to long-lost grandma Shirley MacLaine and the whole family begins a stagy group healing process, the whole exercise feels hopelessly shallow and artificial. In Her Shoes is basically a double-date romantic comedy, in which not one but two women find themselves and learn to live and love again, etc. etc., and while it's well-acted on most counts, it's also as plodding as it is obvious. It's a rare film that requires a trash-talking best friend and a lively, smart-mouthed, comically blunt little old lady to liven things up. It's an even rarer film that can't be helped by either one.