Comedy legend Bob Newhart heads to CBS' The Big Bang Theory on Thursday with a role that was a long time coming. Co-creator/executive producer Chuck Lorre had been courting the Newhart icon for years, constantly going after the six-time Emmy nominee for a role on one of his shows before coming up with the winning role: Professor Proton, an influential character from Leonard (Johnny Galecki) and Sheldon's (Jim Parsons) childhood.

Before accepting the role, Newhart laid out two conditions that would bring him to his first half-hour comedy gig in years -- it had to be for more than a one-off and his scenes had to be performed in front of a live audience, the only way Newhart says he operates.

The Hollywood Reporter caught up with Newhart to discuss his comedic return, the appeal of TV's No. 1 scripted comedy and what the future holds for the good professor.

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The Hollywood Reporter: The Big Bang Theory marks your first half-hour comedy appearance in some time. Why this show?

Bob Newhart: I've been approached for other projects -- some pilots -- and that's too much. Emotionally, I couldn't go through another pilot to series, it takes too much out of you and it's for the younger people today. When I look at a show, I look at it from a writing standpoint. I've said all along that I stumbled on the formula of an incredible cast and great writing, and then take all the credit myself -- it's worked in three different permutations. I know Chuck Lorre personally and am familiar with his work­­ going back to Roseanne and Cybil. The Big Bang Theory is well written and it has an incredible cast. It's intelligent, which is a vanishing commodity. It's setup joke, setup joke, setup joke. When we were doing The Bob Newhart Show and Newhart, we'd take a minute to set up a joke, but it paid off because the reaction was huge. But there seems to be a rhythm now in sitcoms -- setup joke, setup joke, setup joke. As a result, the audience doesn't know what the joke is but they know it's coming because that's where it always goes. There's an internal rhythm. That's one of the reasons I took The Big Bang Theory.

THR: Lorre has said he's been pursuing you to appear on one of his shows for a long time. How did this guest gig come to pass?

Newhart: Chuck was on the lot when we were doing The Bob Newhart Show 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 The Big Bang Theory. He said to me fairly recently, "What would it take for you to do one of my shows?" There aren't too many producers who can use that phrase. He suggested Big Bang Theory 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 Newhart, Lucy, Honeymooners, Mary Tyler Moore, and All in the Family 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. He was fine with both and later called me with the story of Professor Proton. In my era, it was Don Herbert -- he was Mr. Wizard and he did all these experiments for kids with common things around the house. It turns out that Sheldon and Leonard grew up on that show, and that made them want to become scientists. They found out that Professor Proton still does children's shows and they hire him to come and do what he thinks is a children's show, but it turns out to be a show for these two grown people. They're both very enthusiastic and Penny is amazed by one of the things he does.

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THR: Walk us through your day on the set. What was it like?

Newhart: I asked Chuck, the writers and our director if they were going to announce I'm in the show before the taping. I was a little nervous that the live audience wouldn't recognize me and there would just be silence. They said, "We'll come up on you," and thankfully they recognized me; they applauded and stood up. That was very nice. It's amazing how close the set resembled Stage 17 at Radford, where we did The Bob Newhart Show and Newhart. It was like going back in time. It was a relief. You never presume anything. At least in comedy, you get that beaten out of you. You may have done 20 great shows in a row and come to one, and it doesn't work. You never presume anything. After the taping, they had me come out last and the audience all stood up and applauded. That was it until I went in later to tape a different end tag. It was a great experience and took me back in time. I don't know what's happening because us old codgers -- Betty White, Mary Tyler Moore, Cloris Leachman and Valerie Harper -- we're having a revival, I guess!

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THR: Did you realize how much you missed it? Did you feel similar pressure?

Newhart: I'm realistic. I have no desire to go back on a weekly basis. It's for young people. Emotionally, it's very tough. I've always said people say on a dramatic show, "I was crying. It was so emotional when he went and grabbed that little girl from a burning building and handed her over to her mother." In comedy, the best thing you can say is, "I think it's funny." There's a tension and adrenaline rush when you're doing it in front of a live audience.

THR: How has comedy changed since The Bob Newhart Show?

Newhart: There's a rhythm with setting up jokes, which the audience wanted. I don't know which came first -- if the producers and writers decided that was way to go and audience went along with it or if audience wanted it to go that way and producers and writers realized it. Obviously, it's changed in terms of the material you can deal with. I loved Everybody Loves Raymond. It was a beautifully cast show. They dealt with material that Standards and Practices would have said, "This is a joke. You're not really going to do that, are you?" That's the nature of comedy. Today there are fewer and fewer sacred cows. In fact, I don't who the scared cows are anymore or if there are any because the networks have to compete with cable and cable doesn't have those restrictions.

THR: While ratings are down across the board, Big Bang Theory continues to grow -- a rare feat today. What is it about this show that works?

Newhart: 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. When we were on -- along with Mary Tyler Moore and the murderer's row of All in the Family, M*A*S*H and Carol Burnett -- we'd get almost Super Bowl numbers and there were only three of us. We'd get 44-45 shares, but there was some competition from cable. Now I look at the ratings and it's the first time I saw that No. 6 was The Bible on the History Channel. I've never seen a cable show that high in the national ratings. And people find the shows they like.

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THR: You've earned six Emmy nominations in your illustrious career. Is it your hope that Big Bang be the lucky seven that you end up winning?

Newhart: (Laughs) They want me to put my appearance in the show up for a guest performance and they feel strongly about this particular episode. I will.

THR: Will you return to Big Bang?

Newhart: It's a recurring part. I'll probably do two episodes next year.

THR: Would you want to do more than that?

Newhart: 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.

The Big Bang Theory airs Thursdays at 8 p.m. on CBS. Are you looking forward to seeing more of Professor Proton? Hit the comments below with your thoughts. Check out a promo for his appearance, below.

Email: Lesley.Goldberg@thr.com; Twitter: @Snoodit