Actors professionally ‘pretend’ every day for a living. This means it can be hard to tell if an on-screen chemistry is for real or something just put on for the cameras. The American sitcom The Big Bang Theory entered the world’s living rooms in 2007 and soon fans everywhere were graced with four nerdy friends, following their professional and personal lives. Critically and publicly acclaimed, the show has won seven Emmy Awards from 46 nominations, with millions of viewers and one spin-off series. It became a slam-dunk hit that popularized nerd culture.

Working together for 12 seasons and 279 episodes, the characters on the show spent a lot of time together in real life, becoming a close circle. Arguably, one of the most prominent relationships featured on the show lies between eccentric Sheldon Cooper and his often-frustrated roommate, Leonard Hofstadter. The chemistry, conflict, and genuine affection between these two characters were a part of what made this show such a success. The characters who grew together in love and friendship on-screen also supported each other for over a decade off-screen. Here are 15 lesser-known facts about Johnny Galecki’s and Jim Parsons’ off-screen friendship!

15 Bonding Over A Shared Role

When two people are in competition for the same role it can be awkward. Thankfully, after Galecki was offered the role of Sheldon first, he turned it down because he liked the challenge of playing straight man Leonard. If Galecki had taken on Sheldon Cooper, who knows who would have been cast to play Leonard, or if the show would have become a big hit.

14 Becoming Besties

From the very first episode, there was a spark and a respect for the talent between these two leading men. Express reported that Jim Parsons said, “I knew instantly, and I couldn’t tell you even now exactly what it was, what quality, but I know how I felt when I read with him which was very free, there was something so independent about what he was doing. I literally felt it from the very first time we read through it together, it was like ‘well this is different’” The chemistry both on and off set just grew from there.

13 Shared Sad Family Stories

Sometimes it’s sharing an experience that bonds people together. Both Jim Parsons and Johnny Galecki lost their fathers in car accidents. For Galecki, this happened when he was just 16 years old, for Parsons it happened just a couple of years before he was cast on The Big Bang Theory. Sharing this type of experience and working through it together is what can bring a friendship even closer.

12 Who Are Still Friends Now That The Show Is Over?

While everyone on the show has worked well together, naturally some are closer than others. Galecki and Parsons are going to remain friends, but there are other castmates who also mean a lot. Parsons said, “I’ll particularly miss Simon Helberg; we spend a lot of time together, we go back and forth to our dressing rooms, we’ve talked for almost 12 years now.” Galecki and Mayim Bialik first met in the 1990s when he appeared on her show Blossom as a love interest, and that friendship grew over the run of Big Bang.

11 Who Attended Who's Wedding

Celebrities need to be careful to make sure their weddings don’t become a media circus. Coincidentally, Jim Parson got married just a few days after Sheldon Cooper popped the question to Amy in season 10. While the photos of the wedding weren’t plastered everywhere, cast from the show were present. During an interview, Mayim Bialik commented that the wedding was, “stunning and beautiful” but kept her lips sealed on other details, including who else was in attendance.

10 Annoyances?

The cast of Big Bang knows that the show about lovable nerds isn’t for everyone (even though millions of viewers watched it live and continue to watch it in syndication). Vulture reported that Jim Parsons said, "People have so many choices on TV now, so no one’s asking for you to marry us. You can enjoy our show without a weekly appointment.”

9 More Than On Air Chemistry

Both Galecki and Parsons have mentioned that everyone was crying during the final episode taping. The hard work and friendship that has gone on during the run of the show will last a lifetime. While acting is in the bag for the cast, showcasing scientific knowledge over the years doesn’t come that easily. Jim Parsons often looked up terms used in his lines, online, to make sure he understood them. He also said, “I do note cards all week. And in longhand, I write out all my lines again and again and again. And on the weekends, I drill them.”

8 The Mario Lopez Bromance

Outsiders on the set can change the dynamic of things. When Mario Lopez was invited to spend some time on the set of Big Bang for Extra, not everyone was loving his interview techniques. Mayim Bialik was not a fan of the questions about her love life, and Jim Parsons wanted to leave the conversation where Lopez and Galecki ‘broed’ it up over cologne scents.

7 Changing Together Through 12 Years

With a cast that has remained consistent, with a few regular additions since the start in 2007, it’s no wonder they’re all tight. In talking about how the characters have evolved, Jim Parsons told The Hollywood Reporter, “Chuck Lorre once said that watching these characters change will be like watching paint dry — you wouldn't even notice it was happening. That turned out to be both true and not true. We got to play long enough that they were allowed to change and you did notice it because at some point they were writing enough episodes, like, well we have to. They have to morph a little bit. And that made it really fun. I'm deeply satisfied with how it ended."

6 Relationship Drama?

For two years, co-stars Galecki and Cuoco dated but worked hard to keep it under the radar. Cuoco said, “It was a wonderful relationship, but we never spoke a word about it and never went anywhere together." The two didn’t let the breakup impact their professional relationship. Galecki told said, "We're dear friends, still. Kaley's not just an ex, she's a part of my life." He also noted in another interview that one of the reasons they don’t talk much about it is they don’t want it to cloud people’s opinions of the Penny and Leonard relationship.