How 'The Big Bang Theory' scored Billy Bob Thornton for a surprise guest spot

The Big Bang Theory moved back to Thursday night with nothing short of a miracle: The show was able to keep a surprise cameo under wraps ahead of airing, an act that’s nearly unheard of in an age where spoilers freely leak long before episodes debut.

The CBS comedy welcomed Oscar winner Billy Bob Thornton, who played Dr. Oliver Lorvis, a urologist who misunderstands Penny’s (Kaley Cuoco-Sweeting) flirtation while working and attempts to woo her. How did the cameo come to be?

“A few months ago, we had seen an interview with Billy Bob talking about shows he likes. He was saying how much he likes Big Bang Theory, and watches it all the time, and gets so into it [that] he starts talking to the characters on the screen,” showrunner Steve Molaro tells EW. “We thought that was so cool, and we think he’s so great.”

The producers began with the idea of Penny taking off her engagement ring while working as a pharmaceutical rep to boost her sales. “When we started thinking about Billy Bob, the thought was that he could be the doctor who doesn’t quite understand the signals—that this would be a socially awkward doctor who falls for Penny’s sales flirtations. We went back to Billy and his people with that thought, and they thought it was really fun and cool, so we went from there,” explains Molaro.

Surprisingly, the producers didn’t even have to bribe their live studio audience not to reveal the news on social media in the short turnaround between filming and airtime. “We knew that the network wasn’t going to promote it, but there’s only so much you can do, audience-wise. But somehow it stayed a secret,” Molaro says. “That was what we were hoping for, and it worked out well. People really responded to him and the episode.”

Will the Fargo star return in a future episode? Though there’s nothing set in stone, Molaro says Big Bang would welcome him back with open arms. “He has said he had a wonderful time,” the showrunner says, noting that Cuoco-Sweeting and Thornton bonded over stories about fellow former co-star John Ritter. “All of us just fell in love with him. It was a magical week. If it can work out, we’d love to have him back. After we finished taping, we had a table read the next morning. Everyone was a little sad that Billy was gone. It was that much fun having him around.”