It isn’t hyperbole to say that Amanda Peet gives the performance of a lifetime in “Fantasy Life.” There probably shouldn’t be any confusion between the actress and the role she inhabits as Dianne, a one-time screen star now in her early fifties who stopped booking parts a decade ago when the material wasn’t up to snuff. But there’s a wisdom and weariness to suggest Peet’s seen some things, and it can be unnerving to realize that it’s been about as long since she has been on the big screen herself. Her luminous return in Matthew Shear’s lightly comic drama about two people at a crossroads reminds that her fearlessness has been sorely missed as she throws any vanity out the window to play a walking tangle of anxieties, despite living quite comfortably with homes in New York and Martha’s Vineyard.

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