The romantic comedy recipe is so well-known and foolproof that the great mystery about them is how rarely the romantic fools in Hollywood get it right.

"What If" does. It’s a healthy serving of great "obstacles to romance," generous helpings of cute, alluring leads, a dash of funny-sexy "best friends," an enticing location filmed at its best and topped with bright, witty (but not precious) banter. It and its adorable stars — Daniel Radcliffe and Zoe Kazan — have us rooting for them from first frame to last.

Wallace and Chantry "meet cute" at a party. He’s creating faintly forlorn poems out of fridge-magnets, she reshapes them into more hopeful odes to love.

He’s 379 days past his last, brokenhearted breakup. And her? She’s beguiling, an animator with big eyes, easy to talk to. They chat about the sandwich that killed Elvis — "Fool’s Gold" — and how much of that confection was found in the King’s colon.

"We should hang out," she says as he walks her home. Great! Then, as she’s fishing for her keys, "My boyfriend will be worried about me."

There’s the rub. She’s in a five year live-in relationship with a handsome, successful U.N. copyright attorney, Ben (Rafe Spall). Wallace is perplexed. Naturally, he turns to his hipster-Lothario best friend, Allan (Adam Driver, sort of "Justin Long: The Next Generation."). What’s Allan’s rule?

"If it starts dirty, it ends dirty." In other words, even if Wallace attempts to win Chantry — And what kind of name is that? — luring her away from a relationship means getting her to cheat. If that’s how they begin, that’s how they’ll end.

‘WHAT IF’ Rated: PG-13 Opens Friday

"What If" — a limp title for a movie based on the Canadian play "Toothpaste & Cigars" — is about Wallace’s longing, Chantry’s winsome doubts and the other people’s efforts to interfere with what might be happening as they attempt to "just be friends."

Everybody in this is amusing, but not stand-up comic amusing. The cleverness feels unrehearsed and spontaneous. The first time Wallace meets Ben, he’s come over to dinner at their apartment and Ben stops chopping vegetables to wave his knife at Wallace and ask, "Are you trying to sleep with my girlfriend?"

Allan hooks up, for good, with Nicole, played by the willowy and edgy Mackenzie Davis ("That Awkward Moment"), and they animate every scene they’re in with a groping/kissing/pillow-talking hilarity. Megan Park scores snarky points as Chantry’s on-the-make younger sister, who sets her eye on Wallace because Chantry, plainly, is out of bounds.

Kazan, as she proved in "Ruby Sparks," has a whimsical, quirky girl-next-door appeal. Radcliffe, wearing post-Harry Potter stubble and delivering toothy, jaw-jutting grins, makes it easy for us to believe he cannot get her out of his head.

As they "hang out/not date" through the many charming corners of Toronto, neighborhoods where 20-somethings can gather for knitting parties in a yarn shop, or skinny-dip in the lake, "What If" keeps us guessing. The only real give-away is the walking sight gag these two are. Director Michael Dowse surrounds them with people who tower over them. Spall’s Ben has to bend over at the waist to keep his girlfriend, and his girlfriend’s "friend," in the frame and in eye contact.

But the recipe, fools slowly rushing in while joking about Fool’s Gold, works. It’s every bit as irresistible as that potentially lethal combo of bacon, butter, jam and peanut-butter ever was.

Review by Roger Moore, McClatchy Newspapers