Sarah Michelle Gellar, the stake-wielding star of ''Buffy the Vampire Slayer,'' and her fiancé, Freddie Prinze Jr., Mr. ''She's All That,'' are as mismatched as grilled-eggplant-and-chocolate-syrup sandwiches -- the favorite meal of Shaggy, the scruffy sidekick in the couple's latest collaboration, ''Scooby-Doo.'' Gellar wakes before 6:30 a.m.; Prinze can doze until noon. She reads Oprah's Book Club selections; he devours comic books. She loves to travel; he kissed the ground upon returning from the five-month-long ''Scooby-Doo'' shoot in Australia. Her series is humming along; after working the teen-romance circuit, he's in search of a new thing.
''When we were doing 'I Know What You Did Last Summer,' I would have never thought of those two together,'' says the film's producer Neal H. Moritz. ''I just found them to be so different. But when I see them out or at dinner, they just genuinely care so much about each other.''
In a town filled with celestial couplings -- and the tabloid tales that seem to inevitably follow -- Gellar, 25, and Prinze, 26, who became friends while costarring in the aforementioned 1997 horror flick, have a reputation for being sickeningly harmonious. ''He's completely in love with her,'' says Prinze's ''Down to You'' costar Rosario Dawson. ''When they got together, he was blown away.... He was thrown by how hard he fell.''
And he still hasn't touched the ground. Over lunch at a dim L.A. restaurant, Prinze recalls the filming of ''Scooby,'' most of which took place in Queensland on Australia's Gold Coast. ''If Sarah wasn't there, I would have cracked up,'' he says, hunched over his second cup of coffee. ''If I don't see her for a day, that's not a very good day.'' He straightens and reaches for a slice of goat-cheese pizza. ''That's my girl, so I gotta hang out with her.'' (Insert collective ''Awwwww'' here.) ''If we were lonely for home, at least we had to be lonely together. It was a long, long shoot.''