EXCLUSIVE! How 'Big Bang Theory' Surprised Fans With Billy Bob Thornton's Guest Spot!

Did you catch the surprise last night?!

Fans of The Big Bang Theory were floored when a super-secret guest star randomly popped up on the series' first episode back to Thursday nights: Billy Bob Thornton!

That's right — the Academy Award-winner stopped by TV's most-watched comedy for an unpublicized appearance as Dr. Oliver Lorvis, a urologist who misunderstands Penny’s (Kaley Cuoco) workplace flirtations and attempt to woo her at her apartment.

So how did the powers-that-be at BBT keep such a monumental guest a surprise? Executive producer Steve Molaro exclusively spilled the details to ETonline. "It's crazy—it actually worked!" he exclaimed Friday morning.

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"It started with Billy and his people saying, 'Hey, what if we kept this a secret?'" Molaro revealed. "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."

Molaro spilled that the thought of having Thornton on the show has been on their wish-list for a while. "Billy had done a red carpet interview saying that he loved Big Bang Theory and that he watches it all the time, and we said that's pretty cool — if he likes the show, maybe he'd do it," he said.

The producers had been toying with the idea of Penny taking her engagement ring off at her new pharmaceutical rep job as a way to flirt her way into higher sales, and their creation of Thronton's clingy character stemmed from there. "We thought, 'What if he's the doctor who doesn’t quite understand the signals she's putting out and pursues her?'" he said.

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The EP continued, "[He could be] socially awkward and sympathetic and ultimately a good guy who just doesn't quite get it, and Billy and his people all thought that was really fun, so with that we wrote the script."

Since Thorton's appearance was kept completely hush-hush, Molaro said it's been fun to see audience's shocked reactions on social media. "People seemed to really enjoy the surprise of it and really enjoy him in it," he said.

But if you thought this was the biggest shocker of the season—thing again. "We're gonna come back strong," Molaro teased. "I would suggest that you and everybody check out [next] Thursday night's episode. I'm crazy excited about it."

The Big Bang Theory airs Thursdays at 8 p.m. on CBS.