Marking his first major drama-series gig, John Leguizamo is joining the cast of ER this season, TVGuide.com has learned. Details about his role could not be immediately confirmed, but sources say the actor-comedian will play a new doctor at County General in at least 12 episodes beginning in late October. ER producers have been on the prowl all summer for a big-name actor to fill the void left by Noah Wyle. Their first choice, John Stamos, was forced to nix the gig after ABC went and renewed Jake in Progress for a second season. Turns out that wasn't a joke; they really did renew it.