Andy Cohen ended up on Riverdale because he wanted it to happen, and Greg Berlanti listens to his radio show
Source: E! Online
"I'm a huge Riverdale fan, and I was on (SiriusXM's) Radio Andy one day having just watched the season one finale, and it hit me, I was like, I need to come to Riverdale," Cohen recalls. "I want this to happen. So I said it on the air, and (executive producer) Greg Berlanti listens to Radio Andy on his way to work. And he emailed me that day, he said it's done. It was pretty easy."