“Max Rose” is built around an idea: What if Jerry Lewis, the antic but aging comedian, played sad? In this movie’s title role, Lewis plays a jazz pianist who discovers that his recently deceased wife may have had a lover. Written and directed by Daniel Noah, “Max Rose” intends to show Lewis as-is — snowy hair, jowls, slouchy socks and all.

It’s an idea, all right, but that doesn’t mean it’s a movie.

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And the idea isn’t even terribly original. Lewis already famously played a non-comedic role, the slick television star Jerry Langford in “The King of Comedy” (1983), and a weepy role you’ll never see (at least not legally) in “The Day the Clown Cried,” a concentration camp drama that Lewis buried out of embarrassment in 1973. “Max Rose” may not be as bad as that, but it’s a mawkish, morose, heavy-handed affair built around a threadbare, simplistic story. What a shame that “Max Rose” marks Lewis’ first substantial film role in nearly 20 years.

It isn’t news that Lewis can act, but Noah seems stunned by the discovery. His camera constantly lingers on Lewis as if to prove that, yes, the guy can hold a straight face. That’s about the only face Lewis ever holds, since Max is such a dourly written character. Max is haunted by his failed career; angry at his grown son, Christopher (Kevin Pollak); and only half-responsive to his doting granddaughter, Annie (Kerry Bishé). When Max finds that his wife’s silver compact bears a hidden inscription, he has one more thing to brood about.

What follows is a vague character study punctuated by flashbacks featuring Claire Bloom as Max’s wife, Eva. The only time Lewis springs to life is when Max falls in with several rest-home hipsters played by Rance Howard, Lee Weaver and the Beat-era comedian Mort Sahl. Their improvised, late-night bull session, fueled by jazz and booze, is such fun that you’ll wish Noah would just scrap the rest of his script.

Instead, “Max Rose” keeps going, drawing little parallels to Lewis’ own life (the showbiz past, the medical issues, the strained father-son relationship) instead of telling a fully formed story. That’s too bad. For Lewis, who turned 90 in March and now uses a wheelchair, every movie should count.