This must have been the easiest movie to sell: Barbra Streisand and Seth Rogen as mother and son. The poster alone for "Guilt Trip" is irresistible — two of them crammed into a small car, Mom driving Sonny crazy and piling on the guilt with a steam shovel.

That sounds fun, but the actual movie is different, and better. "Guilt Trip" is not some extended Hollywood sketch about mother-son relationships, and if you expected some stereotypical riff on Jewish mothers, forget it. Rogen and Streisand’s name is "Brewster" in this, and they’re not identified in terms of any specific religion or culture.

“GUILT TRIP” Rated: PG-13 Opens today

The title notwithstanding, there’s not even much guilt to speak of, certainly none the mother is dishing out. Instead we have something rare, a warm and insightful comedy about a mother and her grown son.

The characters are rendered in specific terms, yet many people will doubtless recognize elements from their own lives. Streisand plays the kind of mother who, if her son were Lee Harvey Oswald, she’d find a way to blame Kennedy.

Her belief in him is so complete and unshakable — and to him so irrational and baseless — that he takes her for granted, maybe even thinks she’s a bit thick. He doesn’t quite realize that she is the water in his psychological aquarium.

Rogen plays an organic chemist who devises a 100 percent organic cleaning product that is so safe you can drink it. In an early scene we see him pitching it to the board at Kmart, getting flustered and bogged down in corny jokes and scientific jargon. Here, as always, Rogen is not a cheap-laugh guy, but a truthful comic actor, so the scene is funny but painful, too: He really has something wonderful to sell, but he’ll never sell it this way.

Most of "Guilt Trip" takes place over the course of a long driving trip from New York to San Francisco. The son has arranged a series of pitch meetings across the country, but he also hopes to surprise his mother, a widow, by reintroducing her to her first beau, who lives in California. So he invites her to come along. There are laughs throughout but "Guilt Trip" isn’t joke happy. The humor is light and well observed, as when Mom keeps playing the audiobook of "Middlesex," and the son gets uncomfortable hearing about anything sexual in front of his mother.

The real appeal of "Guilt Trip" is not in the bits, but in the interaction between Streisand and Rogen, who are completely convincing as mother and son. Maturity has released something in Streisand, who has a new warmth. When she looks at Rogen, you know that she sees him — everything good in him, everything lacking, everything ridiculous, all his potential and all his history. You believe that she used to change his diapers.

You also believe Rogen, who has the real journey of discovery in "Guilt Trip." What a lucky guy: His mother thinks he’s great, so he’s always going to think he’s great, if only deep in his core. His mother is loving, so he’s always going to be loving and expect love from women. Far from a nuisance, his mother is his edge in every possible way one person can help another person.

"Guilt Trip" is about a son’s coming to realize those things, and how sometimes that takes years.