Cast of CBS hit going out with ‘Bang’

Kaley Cuoco’s unreserved laughter rings through the cool February air in Culver City, California, where she stands alongside “Big Bang Theory” co-star Johnny Galecki during a magazine photo shoot.

The beloved hit CBS comedy, about a group of science nerds and the women who love — and challenge — them, will air its 279th and final episode Thursday.

Cuoco’s upbeat mood is helping to keep sadder feelings about the show’s end at bay … for now.

And to think, one of the most popular series in history almost didn’t make it on the air. A failed pilot for the 2006-07 season starred Galecki as nebbishy experimental physicist Leonard Hofstadter and Jim Parsons as his prickly roommate, theoretical physicist Sheldon Cooper; nobody else from the current cast was involved.

The pilot was retooled, adding Cuoco’s struggling actress (now a pharmaceutical rep) Penny (her last name has never been revealed), plus the roommates’ equally geeky friends, aerospace engineer Howard Wolowitz (Simon Helberg) and astrophysicist Rajesh Koothrappali (Kunal Nayyar).

“Big Bang” premiered on Sept. 24, 2007, and quickly built a fan following. The sitcom peaked at an average of 20.4 million viewers for the 2015-16 season, earned 52 Emmy nods (with four outstanding lead actor in a comedy wins for Parsons) and launched a hit prequel (“Young Sheldon”).

So how did a show that made nerds cool become such a phenomenon?

“‘The Big Bang Theory’ uniquely brought tens of millions of people together every Thursday night,” says Kelly Kahl, president of CBS Entertainment. “I think the longer the show ran, the more the audience saw of themselves in the characters.”

As the end nears, the gang looks back

Jim Parsons: I recall feeling like I was walking on a tightrope for that first audition. The dialogue was so dense — to me, at least — and had lots of multisyllable words. I had never met Johnny, (but) as soon as we read through the scene, I thought he was the one. I just felt it. Thankfully, everyone else agreed.

Peter Roth, president/chief content officer, Warner Bros. Television Group: In the first pilot, the character of Penny (originally named Katie) was not as appealing as that proverbial girl next door. It was not the actress (Amanda Walsh) but rather the conceit of the character. Fortunately, Nina Tassler, then president of CBS Entertainment, realized we had something very special and said, “Let’s do it again.”

Kaley Cuoco: I didn’t get (the role of Katie) the first time around. I was too young, which I love saying because I don’t get to say that I’m too young anymore. (Laughs) Then, a year later, I heard they were doing it again, and they brought me back in to read for Penny.

Bill Prady, co-creator/executive producer: The triangle in the pilot (between Leonard, Sheldon and Penny) is more than just romance; it’s the core of the show. You’ve got Leonard with two forces pulling at him — Sheldon, who is pulling him away from the world, and Penny, who’s essentially pulling him out into the world. She’s an irresistible force.

Johnny Galecki: I really liked the dynamic between Leonard and Penny, and I knew they were going to navigate all those territories of on-again, off-again love. That was clear from the page before I even met Kaley.

Cuoco: As the seasons went on, you could tell that the Leonard-Penny relationship was not going to be conventional, which I loved. It felt very real. There was a crush, they went out, then they didn’t and then they both had different relationships. (They finally married in Season 9.)

Galecki: It felt very natural, almost musical, (to reshoot) the pilot when Kaley was on board with Simon and Kunal. It felt like a band you’d been playing with for years. And everything that everyone did complemented what someone else had to do. It was just fantastic. There’s no recipe for that kind of chemistry.

Parsons: (Adding Raj and Howard) expanded our world, our story possibilities. Midway through the pilot, just when you think you’ve met everyone, in pop Simon and Kunal, and it’s just magical.

Simon Helberg: When Kunal and I entered — and, remember, this was our first time ever on camera as these characters — the (studio) audience erupted, and they didn’t even know who we were! They were so excited that there were more like Sheldon and Leonard. The director had to stop and wait for the audience to calm down.

The ensemble expanded in Season 3 with the additions of microbiologist Bernadette Rostenkowski (Melissa Rauch) and neurobiologist Amy Farrah Fowler (Mayim Bialik) as love interests for Howard and Sheldon.

Melissa Rauch: When Bernadette was introduced, she was sheltered (and had an) overbearing mother, much like Howard.

Helberg: Bernadette cut through Howard’s sleaze like a fine dish soap removing the grease. Melissa and Mayim caused this whole sea change in the show. The layers they brought to Big Bang are really a fundamental part of the show’s success, because they humanized all the characters that much more.

Roth: The introduction of Amy in particular was one of the funniest things I’ve ever seen. To see the perfect realization in female form of Sheldon was hilarious. And Mayim embodied her so perfectly.

Mayim Bialik: I remember being nervous because the cast had been working together for three years, so they had a really good rapport. I only had maybe five lines (in Amy’s first episode), so there wasn’t a tremendous amount to do, but it was really fun.

Parsons: I thought it was such an original idea, the way they brought Amy in. (Raj and Howard had signed Sheldon up for an online dating site.)

Rauch: The female camaraderie among Penny, Amy and Bernadette has played a significant role for all of them. It was during some of their first girls’ nights that we got more of a taste of Bernadette’s sass.

Time to say goodbye

Cuoco: I was watching the finale of “The Middle” last week, and I started crying because it’s such a sweet episode. Then I started getting hysterical, like convulsing. I cried for probably 30 minutes. I was just devastated that we were going to have a series finale, too.

Helberg: I’ve been on the show for almost a third of my life, so I expect closing out this chapter will rock me off my axis a bit.

Parsons: The last 12 years of my life can rest comfortably and compactly under the chapter titled “The Big Bang Theory.” That’s a real blessing, a real gift, to be able to have this view of time passing and to have this moment to come up for air, hit pause for just a brief second and ask yourself: “What am I doing with the next 12 years? They’re not going to plan themselves!”