Behind-The-Scenes Secrets From The Set Of 'The Big Bang Theory'

The original The Big Bang Theory pilot was meant to premiere in 2006-2007. It didn't get picked up by the network, though, and never aired. In the pilot, Sheldon was especially different. According to Fandom's wiki for the unaired pilot: 

Sheldon is significantly different from his series counterpart: he is sexually active, he frequently masturbates (as opposed to his later counterpart Sheldon) and has multiple fetishes, likes dancing and is a bit more social than his counterpart.

And continuing the theme of a more sexualized version of the character, the wiki says Sheldon's proclivities got further explored:

In the episode, it was revealed early that he had slept with Gilda and has a fetish for women with large hind quarters (butts). He has also seen seven women naked, some of whom aren't even his relatives.

Stuart always finds himself in an awkward situation, including in the episode “The Hot Tub Contamination,” when he and Raj end up in a hot tub together. The actor who plays Stuart, Kevin Sussman, had a hard time filming the episode, and not because he had to get so close to his costar. 

Sussman has a deep anxiety about being underwater. The crew had to coach and encourage Sussman through the scene on the set. Sussman said, “I don’t like putting my head underwater ever... a bunch of people had to coach me, you know, ‘it’s going to be okay, we have a lot of people around.’ 

In the tenth season of the show, Howard and Bernadette have a baby named Halley. The newborn girl changes the characters lives drastically, but the baby itself is never seen onscreen. Instead, we just hear her crying off-camera. And Showrunner Steve Molaro says Halley will only ever be heard screaming offscreen.

The true story behind the mysterious choice? It's a tribute to Carol Ann Susi, the unseen actress who played Howard's similarly always-screaming mother. The actor died in 2014, and her character subsequently did, too. Molaro explained, "She is a loving tribute to her grandmother - this is a nice way for us to keep Mrs. Wolowitz alive.” 

Though Sheldon Cooper speaks fluent Klingon and references the Star Trek universe frequently, actor Jim Parsons has never actually seen an episode of the famous sci-fi series.

Another surprise? Though Sheldon frequently references Doctor Who, the actor has never seen that show either. “I have never seen Doctor Who,” Parsons admitted. “I know what it is of course,” he continued, “but I have never watched an episode.”

As the stars of a hit sitcom, it's hard to keep your private life a secret. The Big Bang Theory co-stars Kaley Cuoco and Johnny Galecki, though, maintained a private relationship under everyone's nose. Cuoco and Galecki spilled the beans in 2010, confirming their coupling ended amicably after two years. Everyone, including the people they spent the most time with on set, were shocked. Cuoco stated:

It was a wonderful relationship but we never spoke a word about it and never went anywhere together. We were so protective of ourselves and the show and didn’t want anything to ruin that. But that also made it sad, too. That’s not the kind of relationship I want-I don’t want to be hiding. We couldn’t do anything. It wasn’t as fun as we wanted it to be. Everyone was always asking, and we deny-deny-denied. And I’m like, “Why am I denying this person that I love?” It took a little bit of a toll on me and I think it did for him, too... We knew how to keep it deep inside and never bring it on that set, ever.

In the series, Johnny Galecki's character, Leonard, dresses very casually. In fact, he is almost always wearing a hoodie. But in the unaired pilot, his character wore a dark suit. In the original script, Leonard and Sheldon's fashion is much different - they dress more old-fashioned than in the current version of the show. Leonard's traditional-looking dark suits eventually became the usual costume choice for the character Priya, instead.