Two new roles – Garrett and Castillo – needed to be filled for the production. The actors selected were both "Star Trek" fans. Christopher McDonald was picked for Castillo. "What impressed me about ", Carson remembers, "was that he wasn't just your romantic leading man; he was actually a very impressive actor." Tricia O'Neil was cast for similar reasons; the actress was not the normal "Star Trek" commander type. Carson was also pleased to work with Crosby and Goldberg. The main cast enjoyed the opportunity to play their characters differently. The unusual degree of friction between characters provoked some concern with the producers. Berman, for example, was afraid that the episode was pushing the alternate timeline too far.