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"The Big Bang Theory" moved back to its Thursday slot with a big surprise on October 30. The most recent episode of the CBS long-running series featured Billy Bob Thornton as a guest star, which was unheard of until the episode aired.

The "Fargo" star played Dr. Oliver Lorvis, a urologist who misunderstands Penny's (Kaley Cuoco) workplace flirtations and attempts to woo her. On how the cameo came to be, Molaro shares, "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." He adds, "And we said that's pretty cool - if he likes the show, maybe he'd do it."

Molaro goes on telling EW.com, "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."

Molaro says it was Thornton's idea to keep his cameo a secret, but he didn't expect that the audience who watched the taping would've stayed mum. "It started with Billy and his people saying, 'Hey, what if we kept this a secret?' " Molaro says to ET Online. "And we thought, 'That sounds really cool!' CBS obliged, but we didn't quite know what to do with the studio audience-it was 200 people who saw it happen. But somehow people kept quiet enough that it was news to many viewers, and we were thrilled that it went down that way."

Thornton apparently had a good time filming his secret cameo as Molaro claims, "He has said he had a wonderful time." The show's crew and cast "fell in love with him," Molaro says, and they would "love to have him back." He adds, "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."