Oscar Levant, the troubled midcentury musician and wag, often said he’d erased “the fine line between genius and insanity.”He says it again, or a version of it, in “Good Night, Oscar,” the unconvincing biographical fantasia that opened Monday at the Belasco Theater. But on the evidence of the character as written, and especially as impersonated by Sean Hayes in a gloomy if accurate performance, Levant doesn’t erase the line so much as fudge it. Certainly the play, by Doug Wright, fails to make much of a case for the genius part of the joke. Instead, it offers a spray of Levant’s most famous quips, like the one about Elizabeth Taylor: “Always a bride, never a bridesmaid.” And instead of dramatizing how marvelous Levant was, it just says so repeatedly. Mostly it’s just a cry; Levant doesn’t seem brilliant but ill.

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