Here’s a surprising revelation for “Big Bang Theory” enthusiasts — some of the most hilarious moments set at The Cheesecake Factory were completely ad-libbed. According to the cast’s insights in Jessica Radloff’s book “The Big Bang Theory: The Definitive, Inside Story of the Epic Hit Series,” they typically adhered to the script because the writers did an excellent job of crafting and safeguarding their character arcs. Nonetheless, occasional improvisations still led to significant, humorous, and even storyline-altering scenes. Subtle actions, like a spontaneous gesture eliciting laughter or spur-of-the-moment decisions about the characters’ antics, became pivotal moments in the show. In fact, one of the series’ most iconic running jokes originated from an initially improvised scene.

We’ve gathered some of the finest actor-originated moments for this article. Collectively, they contributed many memorable laughs to the “Big Bang” saga throughout its extensive duration. Here are 11 top unscripted moments that either transformed the storyline or simply made viewers chuckle in “The Big Bang Theory.”

Wil Wheaton was a recurring presence on “The Big Bang Theory,” occasionally (though perhaps not as frequently as fans might recall) guest-starring to deepen his rivalry with Sheldon Cooper (Jim Parsons). Their conflict eventually encompassed the entire Pasadena crew. A particularly notable instance of Wheaton’s arrival to antagonize was in Season 9’s “The Opening Night Excitation,” where they plan to see an early showing of “Star Wars: The Force Awakens.” In this episode, Sheldon chooses to stay home and celebrate Amy Farrah-Fowler’s (Mayim Bialik) birthday rather than join the movie outing, which is fortuitous since Wheaton disrupts the screening in a full Mr. Spock costume. Despite annoying the audience, he assists by calming Sheldon’s friends’ concerns about the film’s merit.

Interestingly, the cast and crew were unaware of Wheaton’s surprise Spock ensemble. He concealed the attire — including the Vulcan ears — from everyone. “Nobody knew that was going to happen,” Wheaton shared with the Daily Express. “Only some writers were in on it, no studio personnel or crew members were clued in. So when I emerged, the uproarious laughter from the cast and crew was absolutely genuine.” Wheaton understandably saw it as a major triumph. “When you make the crew laugh like that on a sitcom, it’s incredibly satisfying. It’s akin to hitting a home run.” This scene has endured fondly in fans’ memories and remains one of Wheaton’s personal favorites.

Imagining “The Big Bang Theory” without Kevin Sussman’s Stuart Bloom seems unthinkable today, yet his character was initially intended for just a single appearance. “I wasn’t aware at the time that it would extend beyond one episode. I thought it was just a one-time guest role, which then blossomed into an entire career,” Sussman confessed to Tulsa World. By chance, one off-the-cuff improvisation expanded Stuart’s role even further.

Initially, Stuart was portrayed as a balanced, easygoing individual; he even briefly dates Penny during Season 2. However, Sussman improvised a softly whispered “I love you” when interacting with Penny (Kaley Cuoco) in Season 3’s “The Guitarist Amplification.” This small act not only altered Stuart’s path on the show but also secured him as a prominent supporting character.

“Usually there is very little ad-lib or improv on that show, but the writers laughed a lot and, after that moment, they started having me back and I noticed that they started having the character of Stuart sort of regress,” he explained. “It was the first time, really, that Stuart went from just being like a normal guy to being desperate.” From that moment on, the character became a vital part of the series — so vital that he’ll soon lead his own spin-off, “Stuart Fails to Save the Universe.” 

“I’m a princess, and this is my tiara!” Fans will always remember “The Shiny Trinket Maneuver,” in which Amy is gifted a tiara by Sheldon as an apology present. Though Amy was dismayed by Sheldon’s selfishness, she simply cannot help squealing over her gift. Her inner princess is immediately activated, and she can’t stop crying out in joy over her delight for the bauble. The piece of jewelry resurfaces multiple times throughout the show’s run, with Amy ultimately wearing it when she and Sheldon win their Nobel Prize.

The initial reaction to the tiara was at least partially improvised by Mayim Bialik, who revealed in a blog entry posted on Kveller that she changed at least some of the scripted dialogue to match her religious sensibilities. “The original line was, ‘Oh my God, it’s a tiara!’ but I don’t like to say God’s name ‘in vain’ or in performance (since my ‘Blossom’ days),” she explained. “[S]o I just sort of vocalized my way out of the ‘my God’ part and no one said anything to me so I kept doing it that way.” She also added more repetitions of the phrase “put it on me” to cover for the audience’s laughter, so when they quieted down, Kaley Cuoco wouldn’t be left with nothing to do. “I love performing the words of our incredible writers. I couldn’t have written that scene if my life depended on it. But I will perform it–and others–for you as if it does,” Bialik concluded.

Whenever there was a serious moment on the set of “The Big Bang Theory” — whether it be Sheldon’s proposal to Amy, their Nobel Prize acceptance ceremony, or the birth of Howard (Simon Helberg) and Bernadette’s (Melissa Rauch) kids — Penny could always be counted on to shed a few tears.

Sometimes they came out in a humorous flood, and sometimes they simply expressed her pride and joy in her friends. It turns out those waterworks were one hundred percent Kaley Cuoco’s, and, since she’s an emotional crier, they were sometimes spontaneous improvisations. The tears were apparently encouraged by the producers as well, since they punched up big moments and offered a different side to the show’s comedy. In fact, Cuoco is specifically credited by executive producer Steve Molero in “The Big Bang Theory: The Definitive, Inside Story of the Epic Hit Series” with adding depth to the series via her emotions.

This, perhaps, isn’t shocking. Cuoco apparently sobbed during the show’s final table read, wept multiple times during the final season’s tapings, and produced waterworks while addressing the show’s audience for the last time with Johnny Galecki at her side. There’s never any shame in showing your true feelings, even while acting, especially since it can sometimes result in great things.

In “The Fish Guts Displacement,” Amy is so thrilled to get extra personal attention from Sheldon during an illness that she continues to pretend she’s sick after she recovers. Dozens of ointment applications and warm compresses later, Sheldon’s super intellect kicks in, and he starts to wonder if Amy’s lying about her health. When Sheldon finds out she’s taking advantage of him, he declares that his father would have spanked him in such a situation, hinting toward George Sr.’s fondness for corporal punishment (a violent side to the character that would ultimately be erased by “Young Sheldon”). She responds to the notion with joy, presuming that erotic pleasures lie before her. The spanking then ensues, to her delight, though Sheldon informs her she’s not supposed to be enjoying her punishment.

Per the cast’s 2013 appearance at PaleyFest (as reported by The Hollywood Reporter), none of that was supposed to happen before the audience, nor was it supposed to take place on screen at all. The improv was purely actor-dictated — and both of them had a lot of fun performing it for an appreciative crowd. “It was one of the hardest things we ever had to do because I found it tremendously amusing,” Jim Parsons said, joking, “All week I was slapping that a**!”

The inimitable Bernadette Rostenkowski wouldn’t be recognizable without her high-pitched voice and distinguished spectacles — and it turns out Melissa Rauch added both elements to the character as spontaneous choices not mentioned in the casting information.

“At the audition, there were just so many people there and I had just gotten off the phone with my mom, who has a very high-pitched voice. So I sort of did it at the last minute at the audition, because I was just thinking of her and I had a pair of glasses in my bag that I threw on,” she explained to First For Women.” [I]t’s very much my mother’s voice, except without the Jersey accent that I lifted out.”

From there, she went on to mold Bernadette into the woman fans came to love. “Part of the joy of that was her voice is a wonderful instrument. It’s like a piccolo on acid,” Chuck Lorre said in “The Big Bang Theory: The Definitive, Inside Story of the Epic Hit Series” (via SlashFilm). “And then, she’s tiny. So those two things juxtaposed with this balls-to-the-wall killer woman who doesn’t even think twice about cutting some moral corners to get things done… it was joyful to watch.”

If you ever got a chuckle out of Penny’s attempts at tending bar at the Cheesecake Factory, then you have this unscripted accident to thank for that long-running storyline. Kaley Cuoco was injured in a horse riding accident while filming Season 4 of “The Big Bang Theory,” severely breaking her leg after being thrown by her spooked horse. The injury was so bad that she nearly lost her limb.

This also forced her to miss filming two episodes of the show while producer Chuck Lorre helped her recover enough to return. When she was well enough to come back to the set, the show needed to hide her cast. Instead of keeping Penny stuck in her bed, the producers planted her behind the bar at her restaurant job, where she was tasked with slinging booze instead of serving food.

While it was a plot choice necessitated by the actor’s compound fracture, the storyline continued from Season 4 through Season 7, when she focuses on being a better bartender before quitting waitressing and bartending entirely to focus on a career in acting and pharmaceuticals. And none of it might have happened had Cuoco not met with such a terrible accident, truly a blessing in disguise.

During the Season 4 episode, “The Boyfriend Complexity,” Leonard (Johnny Galecki) poses as Penny’s boyfriend while her dad’s in town. They’re so good at pretending to be a couple that Sheldon thinks they’re really back together. He therefore spends hours amending their infamous roommate agreement into something that will factor Penny’s presence back into their day-to-day-lives — until Leonard later informs Sheldon that they broke up again.

Having spent all night retrofitting the agreement to cover all of Penny’s annoying quirks, Sheldon responds by flinging the agreement into the air, resulting in a single page resting upon and sticking to his right shoulder and the right side of his head. The audience howls, and Sheldon proceeds to deliver his disgusted reply with the paper still clinging to him.

This was, naturally, a totally unscripted twist of fate. Galecki and Jim Parsons both ended up breaking character and smiling when it happened, as can be seen in the sitcom’s Season 4 gag reel. The take was subsequently salvaged by cutting both actors’ reactions out.

This is definitely the biggest contribution an unscripted improvisation ever made to “The Big Bang Theory” — Sheldon knocking three times on Penny’s door, followed by him calling out Penny’s name. Per series producer Lee Aronsohn, speaking to Jessica Radloff for her book “The Big Bang Theory: The Definitive, Inside Story of the Epic Hit Series” (via ScreenRant), the script simply said that Sheldon was supposed to knock on Penny’s door once. Aronsohn revealed that the joke came up mid-filming, and he was one who pitched it as three knocks.

“I just said, “What if he does this?” And I demonstrated that instead of one knock and Penny opens the door, it was three knocks. It showed Sheldon’s obsessive compulsiveness. People like that like patterns and structure, so we had some fun with Sheldon once that got established,” he said, recalling one scene where Penny answers the door after he’s knocked twice, and Sheldon is frozen on her doorstep, unable to speak. This small suggestion gave birth to a long-term running joke that launched a major character beat for Sheldon that would span the whole length of the sitcom and even affect a plot point on the show’s prequel series, “Young Sheldon.”