Movie stars don't do bicycles. Once you achieve a certain level of stature in Hollywood, you get ferried around sets in a golf cart or a limo with tinted windows. Jennifer Garner may be a leading lady soon--on April 23 she'll headline a film for the first time in "13 Going on 30," an update of Tom Hanks's "Big" for the Y-chromosome set--but she'll never give up her bike. It's cotton-candy pink with bells on both handlebars--a birthday gift from writer J. J. Abrams, who hired her to star in ABC's sexy spy drama "Alias." "I keep meaning to get streamers for it," Garner says. Today, the 31-year-old actress is trying to pedal slowly so a reporter can keep pace on foot as she makes her way across the Disney studio lot to the "Alias" soundstage. "Sometimes we're here really early and the only other people around seem grumpy. So I'll ride by and go--." She rings her bell. Garner giggles. "I don't think it amuses anyone but me."

"13 Going on 30" should tickle a few more funny bones. Garner plays the grown-up incarnation of Jenna Rink, a bra-stuffing kid who's so consumed with penetrating the cool-girl clique that she snubs her lovestruck best friend Matt. After a disastrous birthday party, she makes a wish to skip straight to fabulous adulthood, and it comes true, sort of. Jenna awakens to discover that she's now a high-powered magazine editor--a sneaky, manipulative one, as if such a creature exists--and a complete stranger to her old friend Matt, played by Mark Ruffalo ("You Can Count On Me"). "13 Going on 30," directed by Gary Winick, is not in the same class as "Big." But it's a sweet, tidy execution of a winning formula, and it's all but guaranteed to do exactly what it was built to do: make tween girls smile and turn its star into an A-list (or at the very least, A-minus-list) movie actress.

It is notoriously difficult for TV actors to make the leap into films--just ask Ray Romano--but in some ways Garner already seems like a movie star, even though her largest role to date was supporting Ben Affleck in last year's tepid "Daredevil." (She'll film a spinoff based on her character this summer.) She's won a Golden Globe (for "Alias"), presented at the Oscars and captured the cover of glamorous magazines. So maybe Garner is to be believed when she says dreams of actual movie stardom don't consume her. "If I was 21, I might've had ants in my pants," she says. "But I auditioned for a long time, and now I have a steady situation. I think I've earned the right to say that I'm OK where I am." Anyway, it's not like she can quit her day job. Garner is signed to "Alias" through 2008. And while the series is more cult hit than ratings giant, it's in little danger of being canceled--given that it's more or less ABC's only good show.

But even if a movie career isn't life or death for Garner, she still didn't want to muff her first solo flight, which makes "13 Going on 30"--her first attempt at comedy--a calculated risk. Garner's chief worry about the film is that viewers won't be able to shake their memories of Hanks while watching her. (Our advice: don't even try.) But it's a measure of her versatility that the mere thought of her in such a role--after three years as the butt-kicking, cleavage-boosting Sydney Bristow--doesn't cause whiplash. "Jen has the supercombo of beautiful and goofy," says Abrams. "That's why she's such a winning lead."

The "13 Going on 30" shoot last summer wasn't a picnic for Garner, but that had nothing to do with the movie. Throughout filming, she was coping--publicly--with the collapse of her three-year marriage to actor Scott Foley, whom she had met while guest-starring on Abrams's first TV series, "Felicity." "I grew up being the good girl," she says. "All the Garner girls are. We're a good-girl family. So to think that people were reading something about me that could change their opinion..." She pauses. "I had to learn that you just have to be OK with that and know you're still a good girl. Your mother knows it, your sisters know it, everybody important to you knows it."

Since last fall, Garner has been dating her "Alias" costar Michael Vartan. She politely demurs when the subject comes up--divorce is public, relationships are not--and the two are never caught cuddling on the set. Still, they're not fooling anyone. Vartan is the troublemaker: between takes, he picks up a cylindrical object from a desk and begins behaving, let's say, adolescently with it. Later, he wags his tongue lasciviously at the camera. Standing beside him, Garner tries not to flinch. "The best way to get him to stop is to say 'action'," she calls out to the director. So there's the secret: she's the good girl, and he makes her blush.

Pretty soon, she's blushing again, though this time Vartan is nowhere in sight. After teaming up on "13 Going on 30," she confides, she and director Winick decided to produce an independent movie together. To do it, she created a company called Vandalia Films, a nod to the original name of her beloved home state, West Virginia. But when she went to register the name for incorporation, she found out it was already taken--by a hard-core porn company. "That name is on my credit cards. It's on my business cards. It's on my assistant's business cards." She wrinkles her nose and turns crimson. "I'm embarrassed," she says, whispering as if she's in church. Don't worry, dear, you're in the movies now. No one will ever know.