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Director Marc Webb, working from Scott Neustadter and Michael H. Weber's witty script, stages each scene as a vivid snapshot memory, and his sense of play is boundless. The film leaps in a heartbeat from the furtive glances (and shared fixation on the Smiths) that ignite an office love affair to a rooftop-party reconciliation that plays out, via split screen, in two simultaneous versions (how the hero wants it to be and how it happens) to a morning-after-the-first-sex saunter that evolves, with joyful hilarity, into a musical number scored to ''You Make My Dreams.'' This has to be the first movie ever to give equal props to Morrissey and Hall & Oates.
(500) Days is like a mood ring cued to the ups, downs, and confusions of modern love. It's a Gen-Y Annie Hall made by a new-style Wes Anderson who uses his cleverness for humanity instead of postmodern superiority. None of it would work, though, without such lived-in performances. Deschanel makes the lovely, sensuous Summer just precocious enough to know what she wants without coming out and saying it, and Gordon-Levitt, with his junior Springsteenian chin jut, lets you read every glimmer of hope, pain, lust, and befuddlement beneath his nervy facade. It's a feat of star acting, and it helps make (500) Days not just bitter or sweet but everything in between.