Over The Rookie’s 100-episode run, the ABC procedural has seen its fair share of guest stars, some of which have marked Castle reunions for Nathan Fillion. Though perhaps one of the more memorable guests has to be former Saturday Night Live cast member Pete Davidson. The funnyman notably played the brother of Fillion's John Nolan, Pete, in three episodes. While Fillion and Davidson seem to be good buddies off camera, the former was actually "embarrassed and ashamed" upon meeting his on-screen sibling for the first time.