Since its initial release in 2007, has become one of the most popular sitcoms of all time. The showrunners were ready to keep on making more seasons, which is why fans were shocked to learn that was coming to an end after only twelve seasons. More than , and they were not disappointed!

Over the years, tons of , including Mark Hamill, Stephen Hawking, and even Bill Gates.

Some guest actors left such an impression on the viewers that they became series regulars in no time. The perfect example of this is Bob Newhart, who played Sheldon’s childhood hero Professor Proton. Believe it or not, Newhart only accepted his guest role on under two very specific conditions. Luckily, Chuck Lorre really wanted him to join the show and agreed.

Bob Newhart Loved His Time On The Big Bang Theory

When he was first offered the chance to guest appear on , . But luckily, his concerns were unfounded. “Thankfully, they recognized me; they applauded and stood up,” Newhart revealed to .

Despite his initial concerns, Newhart loved his time on . "It’s beautifully cast, has great writing and it’s intelligent. There was a longing for something intelligent — to credit the audience being intelligent. It doesn’t talk down to the audience at all," the actor and comedian admitted.

When asked whether he’d want to become a regular series rather than a recurring character on , Newhart said: “I don’t know how many legs this character has. That’s up to them. We’ll talk about it if they feel they want more. That’s strictly up to them. I didn’t just want to do a one-shot. I wanted to make it semi-recurring. At this point, that’s what it was. It was very enjoyable. I loved it.” Luckily, fans loved Professor Proton as much as Sheldon did and Newhart got to return to the show on several occasions.

Bob Newhart Had Two Conditions For Guest-Starring On The Big Bang Theory

You may know him as Professor Proton in , but Bob Newhart has been starring in popular comedy shows and sitcoms since the late 60s! In fact, co-creator, Chuck Lorre, was such a fan that he tried to get Newhart to make a guest appearance on one of his shows for years. He didn’t succeed until came around.

“Chuck was on the lot when we were doing and at lunch, he’d come to our stage and sit there. We’ve been going back and forth, and we never could find a project that we could both agree on until ,” Newhart told in 2013.

The only reason why Newhart accepted to appear on was because Lorre asked him, “What would it take for you to do one of my shows?”

To this, Newhart responded with two specific conditions: “He suggested and I had only two things that were paramount: No. 1, my scenes had to be taped live. There’s a tendency to pre-tape a lot of stuff and put a laugh track on it and you lose something. With and it was always done in front of live audience. I always felt that the live audience gives it adrenaline. That’s the only way I function. No. 2, I wanted it to be a semi-recurring role.”

Lorre agreed to both conditions and pitched Newhart the idea of playing Professor Proton. Newhart loved the idea and went on to portray this character from 2013 to 2018, earning his first Primetime Emmy Award with his phenomenal performance.

This One Cast Member Didn’t Like The Fact That Bob Newhart’s Character Was Killed Off The Big Bang Theory

Bob Newhart appeared for the sixth and last time on in 2018. Unfortunately for the fans, Professor Proton was killed off the show. But Newhart returned on four separate occasions as the spirit of Professor Proton. Fans loved to see this beloved character once again, but in the first place.

In an interview with Variety, because he couldn’t find another logical explanation. "I asked him if he had asked for a raise. Because why else would you kill off Mr. Newhart’s performance on our show?" Just like the fans, Parsons greatly appreciated that the writers brought Newhart back as Sheldon’s Jedi mentor.

Parsons added that he appreciated the spirit manifestation twist, "But this spirit manifestation they’ve opened up now is great," he said at the time.