Kaley Cuoco, Johnny Galecki Nailed Their Big Bang Theory 'I Love You' in 1 Take — While Dating in Real Life

LOS ANGELES - SEPTEMBER 20: "The Hot Tub Contamination" -- Pictured: Penny (Kaley Cuoco) and Leonard Hofstadter (Johnny Galecki). Leonard and Penny must separate a quarreling Sheldon and Amy when their cohabitation does not go as planned, and Sheldon threatens to break off their relationship when Amy refuses to adhere to the bathroom schedule. Also, Howard and Bernadette find unexpected guests at their house when they decide to stay home from a planned vacation, on THE BIG BANG THEORY, Monday, Oct. 17 (8:00-8:31 PM, ET/PT), on the CBS Television Network. (Photo by Sonja Flemming/CBS via Getty Images)
LOS ANGELES - SEPTEMBER 20: "The Hot Tub Contamination" -- Pictured: Penny (Kaley Cuoco) and Leonard Hofstadter (Johnny Galecki). Leonard and Penny must separate a quarreling Sheldon and Amy when their cohabitation does not go as planned, and Sheldon threatens to break off their relationship when Amy refuses to adhere to the bathroom schedule. Also, Howard and Bernadette find unexpected guests at their house when they decide to stay home from a planned vacation, on THE BIG BANG THEORY, Monday, Oct. 17 (8:00-8:31 PM, ET/PT), on the CBS Television Network. (Photo by Sonja Flemming/CBS via Getty Images)

Sonja Flemming/CBS/Getty

Johnny Galecki can't help but look back fondly on one particular special moment from The Big Bang Theory's 12-season run.

The actor, 47, and his costar Kaley Cuoco — who he dated in real life from 2008 to 2010 — remembered the first time their characters Penny and Leonard Hofstadter exchanged "I love yous" during the CBS sitcom's sixth season in Jessica Radloff's oral history book The Big Bang Theory: The Definitive, Inside Story of the Epic Hit Series.

"I think we knew at the time. I mean, you play these characters...but that was an incredibly honest moment between Kaley and I," Galecki shared. "We didn't need more than one take."

LOS ANGELES - SEPTEMBER 29: "The Creepy Candy Coating Corollary" -- While Sheldon settles a score with his nemesis, Wil Wheaton, Wolowitz begs Leonard (Johnny Galecki, left) to get Penny (Kaley Cuoco, right) to set him up with one of her friends, on THE BIG BANG THEORY, Monday, Oct. 19 (9:30-10:00 PM, ET/PT) on the CBS Television Network.(Photo by Robert Voets/CBS via Getty Images)
LOS ANGELES - SEPTEMBER 29: "The Creepy Candy Coating Corollary" -- While Sheldon settles a score with his nemesis, Wil Wheaton, Wolowitz begs Leonard (Johnny Galecki, left) to get Penny (Kaley Cuoco, right) to set him up with one of her friends, on THE BIG BANG THEORY, Monday, Oct. 19 (9:30-10:00 PM, ET/PT) on the CBS Television Network.(Photo by Robert Voets/CBS via Getty Images)

Robert Voets/CBS/Getty

RELATED: Kaley Cuoco Says 'I Only Had Eyes for' 'Big Bang Theory' Costar Johnny Galecki: 'This Is Going to Be Trouble'

Meanwhile, Cuoco, 36, added that Galecki still gets "nostalgic about that moment."

The Flight Attendant star echoed her costar's sentiments, saying, "When we did the moment, it was perfect! It was literally a perfect scene."

"But even when we do a scene that's great, we usually always do it one more time so they have an option," she said. "But they said, 'We don't even want an option.' It was pretty cool."

"[Showrunner] Steve Molaro or [director] Mark Cendrowski came over and said, 'We don't need another take unless you want one,' and we said, 'We don't either,'" she added.

LOS ANGELES - MARCH 1: "The Wheaton Recurrence" -- A fight between Leonard (Johnny Galecki, right) and Penny (Kaley Cuoco, left) threatens their relationship, while Sheldon battles Wil Wheaton in bowling, on THE BIG BANG THEORY, Monday, April 12 (9:31-10:00 PM, ET/PT) on the CBS Television Network. (Photo by Robert Voets/CBS via Getty Images)
LOS ANGELES - MARCH 1: "The Wheaton Recurrence" -- A fight between Leonard (Johnny Galecki, right) and Penny (Kaley Cuoco, left) threatens their relationship, while Sheldon battles Wil Wheaton in bowling, on THE BIG BANG THEORY, Monday, April 12 (9:31-10:00 PM, ET/PT) on the CBS Television Network. (Photo by Robert Voets/CBS via Getty Images)

Robert Voets/CBS/Getty

RELATED: Kaley Cuoco and Johnny Galecki's Relationship: A Look Back

After they finished the scene in single take, Cuoco admitted that she had something else on her mind: the cast and crew's first flash mob. The dance phenomenon was created by the actress to surprise producers and writers on some tape nights, according to the book.

"I was like, 'We don't need to do it again! I'm ready to daaaaaance!' [Laughs] Those were terrific memories for the cast and crew . . . and the fans, thank you very much!" she said of the flash mob.

Galecki responded, "They were, but that's what you were thinking of when we had that special moment? The flash mob?" which Cuoco denied.

Over the course of the show, Cuoco and Galecki bonded and gave each other special nicknames. They called each other "Mooks" after the actor received a jersey with the name on the back. He would also call her "Stickerbell" which "she loved, because she's evolved enough to know she's not one," he recalled.

Kaley Cuoco and Johnny Galeck
Kaley Cuoco and Johnny Galeck

Alberto E. Rodriguez/Getty; Jason LaVeris/FilmMagic

RELATED: Marisa Tomei Almost Played 'The Big Bang Theory' 's Penny: 'What a Different World That Would Have Been'

Writer Tara Hernandez remembered how the fan-favorite scene between the pair came to be after she pitched in the writer's room.

"The Leonard-Penny 'I love you' came up really organically when they were having a moment of insecurity of Penny working with this classmate who Leonard was threatened by," she explained.

After receiving some pushback in the writer's room, she recalled explaining, "Leonard's feeling insecure in that moment is because he hasn't gotten that validation that he needs. Leonard needs to hear that vocalized or else he won't be able to relax about this new partnership that she has."

She added, "I remember everyone really jumping down my throat about it, like, No, we're not there yet! And to his credit, Steve Molaro just sat with it, and was like, 'Yeah. I think it's time. I think it's the time for 'I love yous.''"

Never miss a story — sign up for PEOPLE's free daily newsletter to stay up to date on the best of what PEOPLE has to offer, from juicy celebrity news to compelling human interest stories.

The Big Bang Theory: The Definitive, Inside Story of the Epic Hit Series is available now wherever books are sold.