Megan Fox knows what moviegoers expect of her — and it usually isn't the punch line.

But in Judd Apatow's "This Is 40," which opens Dec. 21, the 26-year-old actress most widely known for leaning suggestively over the hood of a Camaro in the first two"Transformers" movies gets an opportunity to show her funny side.

"It's weird to me that people haven't picked up on it yet that I have a sense of humor," Fox said, during a break from shooting on the Pacific Palisades set of the comedy last August. "When you read my interviews from start to finish, it's pretty obvious that there's a lot of sarcasm. The things that get chosen in sound bites, that run and seem literal and sound like they're very shocking, they were usually jokes, but people for whatever reason can't accept the idea that I could possibly have been joking."

Perhaps this time, when she's surrounded by Apatow's stable of comic actors, audiences will understand that Fox, too, is in on the joke.

"This Is 40" centers on the midlife woes of Pete (Paul Rudd) and Debbie (Leslie Mann), the married couple who were on the periphery of Seth Rogen and Katherine Heigl's pregnancy plot in Apatow's 2007 comedy, "Knocked Up." In the new movie, Pete and Debbie are grappling with getting older — and poorer. Debbie's boutique, once a vanity business, is suddenly needed to help cover household expenses, and she counts on the comely Desi (Fox) to lure male shoppers.

It was Mann — Apatow's real-life wife — who originally suggested he cast Fox in something after seeing the young actress host "Saturday Night Live" in 2009. Mann felt Fox wasn't getting her comedic due in Hollywood, and an encounter with a power player while dropping her daughter off at school sealed that impression.

"I was so surprised by how funny she was," Mann said. "She has an interesting attitude that's fun to watch. She's not actually trying to be funny. She just is. And then I had this little argument with a dad at school. He was an agent or something and he was talking about Megan saying, 'Those girls are a dime a dozen.' I got so mad! ... I said, 'You're a dime a dozen!' People don't give her enough credit, just because she's beautiful."

Apatow auditioned Fox opposite Mann, Rudd and Charlyne Yi, who plays Jodi, a fellow employee in Debbie's shop (and another returning character from "Knocked Up"), and Fox got her first taste of the director's signature improvisational style.

"Whatever comes to Judd, he yells it out and you do it," Fox said. "And it's funny or it's not funny. You just keep going. But I know I'm not gonna come up with anything better than the jokes he throws at me."

It's a markedly different style from her "Transformers" director, Michael Bay, with whom Fox had a public falling out.

"Bay will yell out things for you to do with your hair or body or whatever," Fox said. "He yells out instructions while you're in the middle of the scene. Those are supposed to be more serious moments. It's sort of difficult to be in the middle of a scene when someone yells, like, "Say, 'No Bumblebee, we have to get Optimus to the Dagger's Tip now!'" and you're like, 'Ah ... let me think about it first, it's not gonna come out of my mouth the right way.'"