I'm not the only one who thinks Gellar is a figment-it's one of the popular theories among fans. The thing is, it's possible that he's not, but the way all of his scenes were shot, it allows for both possibilities, and I think the episode with Dexter seeing Head!Brian (and seeing him "doing" things, like stab the hotel owner, only for the pitchfork to end up in Dexter's hands) supports that theory.