Like her costars, Sam Rockwell and Martin Freeman, Zooey Deschanel was no aficionado of Douglas Adams' seminal comic novel The Hitchhiker's Guide to the Galaxy when she signed on to be part of the big-screen version, now in theaters.

"I read the book in the fifth grade," she says. "It was kind of a big thing to read Hitchhiker's at that age. Everybody thought we were so sophisticated to be reading it, but because I was 10 I didn't really get a lot of the humor. I thought I did, but I read it again before starting the movie. There's no way I would have gotten this when I was 10!"

In the film, Deschanel plays Trisha McMillan (although she more readily answers to her space name, Trillian), a free-spirited young woman who has to choose between two very different guys. On the one hand, there's Zaphod Beeblebrox (Rockwell), the president of the Galaxy, who's got a big spaceship and an even bigger ego. But she's also got a soft spot for Arthur Dent (Freeman), the bumbling but charming Englishman she met at a costume party back on Earth.

This love triangle was actually invented by Adams for the film. "There are elements of it in the book," explains Deschanel, "but the Trillian character isn't really in the book that much. And when she is, she's always sort of p---ed off at people. She's flying the ship, thinking 'Why is everyone else so dumb?' She's one of the smartest characters in the book, and I wanted to keep that intact. So we were going for a kind of Spencer Tracy-Katharine Hepburn feel with her and Arthur. [They're] two smart people who everyone knows should be together, but they just can't get it together to get together." Got that?

Of course, fans of the book were understandably a little nervous when they caught wind of this new plot development, even though Adams himself had written it into the script. "You worry about the fans, but in the end, you just have to do your best and do what's best for the character," says Deschanel. "Everyone has their ideas, but all we can do is make the best movie that we can and do our best to do justice to Douglas Adams."

The change certainly didn't hurt the film at the box office — Hitchhiker's opened in the No. 1 slot last weekend and grossed another $9.13 million this weekend.

Next up for Deschanel is a role in the romantic comedy Failure to Launch, which stars Sarah Jessica Parker and Matthew McConaughey. She's also contemplating teaming up on screen with her older sister, Emily, who has acted in such films as Cold Mountain and The Alamo. "We're so different," Deschanel says, laughing. "We don't really look alike at all, so I don't even know if people would believe we're sisters. Some people think we look exactly alike and I always go, 'She's a foot taller than me, and we have totally different facial features.'"