Professor Proton returns!

In tonight's all-new episode of The Big Bang Theory, the one and only Bob Newhart returns for his fifth guest appearance on the smash CBS comedy -- and only ET has the behind-the-scenes scoop straight from the 88-year-old legend.

"It's just great coming back," Newhart dished to our cameras when we recently stopped by the set of The Big Bang Theory's 11th season.

We were delighted to see that Newhart was once again dressed as Sheldon's spiritual guide -- a combination of Obi-Wan Kenobi and Professor Proton -- but apparently those robes are not as comfy as they look! "You love it, but I don't love it," Newhart joked.

When asked what it's like to share scenes with Newhart, Big Bang's leading man, Jim Parsons, was quick with his response. "It is really, really unpredictable in a really good way," Parsons said with a smile. "I mean, like, he is solid. I love it. He is so great."

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So what keeps Newhart coming back to The Big Bang Theory time after time? "It's good writing," Newhart replied. "I knew it was going to be good writing and that's, I think, kind of hard to find these days."

"[And] it's done in front of a live audience, which [is what] I always did for my shows," he added. Plus, let's not forget that Newhart won his very first Emmy for Outstanding Guest Actor in a Comedy back in 2013 after his first appearance on Big Bang. 

As for the rest of the items on Newhart's career bucket list? "Well, the presidency," he joked. "I'm 88, so if I didn't work out, I'd only be 92 [after the first term] and they could find another guy."

Bob Newhart 2020!

Catch Newhart's hilarious guest role on The Big Bang Theory tonight at 8 p.m. ET/PT on CBS.