Nathan Fillion has won fans over alternately as Buffy's Caleb, Firefly's dashing Capt. Mal and a hunky Wisteria Lane doctor. Starting Monday, March 9 (at 10 pm/ET), Fillion will appear as Nick Castle, a wise-cracking mystery writer in need of inspiration in ABC's comedic crime drama, Castle.

Fillion and his smoldering costar, Stana Katic, spoke with TVGuide.com just ahead of the show's arrival to dish on their on-screen battle of the wits, what fans will see from Fillion that they haven't seen before, and why they were each drawn to the show.

Plus, will Fillion be teaming up with Joss Whedon again? When the cameras weren't rolling, the actor said he'd sign up for anything the Buffy creator (and current Dollhouse producer) asked him to join.