While others have made cameos in spin-off Young Sheldon, neither Nayyar nor Cuoco have reprised their roles in the seven years since The Big Bang Theory ended. Speaking to Radio Times about their surprise drop-ins, Kevin Sussman, who leads the cast as Stuart, said: “I was just delighted. (L-R) Melissa Rauch, Simon Helberg, Johnny Galecki, Kaley Cuoco, Jim Parsons, Mayim Bialik and Kunal Nayyar in The Big Bang Theory. Penny, with a rebellious streak seemingly no matter what universe she’s in, has formed a militia alongside her Big Bang beefcake ex-boyfriend, Zack (Brad Thomas Smith). Add Stuart Fails to Save the Universe to your watchlist on the Radio Times: What to Watch app – download now for daily TV recommendations, features and more.