Wallace Shawn by Jason Kempin/WireImage.com
Final details have trickled in regarding Gossip Girl's guest-casting of Wallace Shawn.

The Princess Bride's utterly inept villain will play the father of Serena's new beau, Aaron ( Vanished's John Patrick Amedori), as originally reported by EW's Ausiello. It has since been confirmed that Shawn's character, in keeping with the Gossip Girl books, will also emerge as a love interest for Blair's mother, Eleanor.

We've also nailed down Shawn's first air date: His multiple-episode arc kicks off in the heat of sweeps, on Nov. 11. - Matt Mitovich