Doherty, Perry's former Beverly Hills, 90210 co-star, will guest on the Season 4 premiere, but her appearance on The CW series had been in the works before Perry died in March. "Luke and Shannen were really, really good friends," showrunner Roberto Aguirre-Sacasa told ET over the phone last week. "Luke had talked to me on and off over the years about trying to find something for Shannen on the show and when we were putting together this special episode, we kind of wanted to include special people from Luke's life. There was a very emotional, important role and we sent the script to Shannen and she read it and loved it and said, 'I want to do it,' so it's about honoring their friendship a little bit. I know she was really moved when she read the script and immediately said yes." Cole Sprouse said during Riverdale's Comic-Con panel that having Doherty on set was "cathartic." On Instagram, Doherty wrote of appearing on Riverdale: "I am deeply honored to pay tribute to Luke on Riverdale. The care in which this show takes in honoring his memory is beautiful. He is missed. Today. Tomorrow. Forever."