There’s an invasion going on in Neptune. Neptune, Calif., that is — where UPN’s "Veronica Mars" is set.

Guest stars aplenty are visiting the addictive drama: first up is "Xena’s" Lucy Lawless, who stars in Wednesday’s episode — the first one ever directed by series creator Rob Thomas — as an FBI agent hunting for Veronica’s boyfriend, Duncan Kane. Upcoming guest appearances include return visits from Harry Hamlin as accused murderer Aaron Echolls and Charisma Carpenter, an alumna of "Buffy" and "Angel," as newly broke trophy wife Kendall Casablancas. In an art-imitates-life twist, Kristin Cavallari of "Laguna Beach," an MTV reality show set in a wealthy seaside town just like the fictional Neptune, guests Feb. 15 as a cheerleader at Neptune High.

But the casting news "Mars" creator Rob Thomas sounds most excited about involves two cast members from a Fox show. Thomas says Alia Shawkat and Michael Cera, who play teens Maeby Funke and George Michael Bluth on "Arrested Development," will guest on a March episode of "Mars."

"They’re both playing college freshmen," Thomas says, adding with a laugh, that "two 16-year-olds are playing 19-[year-olds], and all of our our twentysomething actors are playing 18-[year-olds] in that episode."

Cera plays a student giving high school senior Veronica a university tour, but Thomas won’t say much about Shawkat’s role. "If I told you what she played, it would really be giving away the mystery."