How has your life changed since The Big Bang Theory took off?
"The Big Bang Theory has completely changed my life. To be blunt, it's been several years since I've had to cash in any unemployment cheques - and that's really nice. I know that sounds mundane, but it's very true.
"I feel very fortunate to have locked in consistent work as an actor, and obviously that works on so many different levels. It's been really rewarding to spend the last few years without the hatchet over my head of, "Oh my God! What's next for me? What on Earth am I going to do to pay the rent now?"
Does it feel good to have some money in the bank?
"Of course! I've been a working actor for many years, but it's not always been successful for me. I certainly struggled in the past.
"Once, I lived in an apartment with a skylight in the bathroom. Every winter, it would snow through the skyline but we got a discount because of it. Thankfully, life's changed a lot since then."
How do you deal with the fame that comes along with working on a successful show?
"I guess it's tough at times because there are certain places you visit where you get constantly recognised for being on the show. That's fun on certain days, but there are other days when you just don't want to be seen by anybody. I take everything day by day, and I see what happens.
"To be honest, it's all good. There's nothing scary or bad about it. As a human being, you know that there are some days when you'd rather not talk to anybody – but I can't really do that anymore without appearing rude."
It must be mind-blowing to walk around and see your face on merchandise like T-shirts and mugs.
"That's certainly disconcerting. It's very odd. It's fun in some ways, but it's strange in other ways. I'm still getting my head around it."
How many times have you been asked to say Sheldon's catchphrase 'bazinga' today?
"I think I've been asked to say it at least two or three times, but not too many. I turned down a couple of the requests, but then I was asked to say it by an Iraq war veteran and that wasn't something I was going to turn down. I thought it was the least I could do in return for his service to my country."
Would you agree that there were less 'bazingas' in the last series of the show?
"Yes, you're right. There were fewer 'bazingas', weren't there? To be honest, I don't remember when Sheldon first said it on the show, but it certainly wasn't intended to catch on. It wasn't intended to be a catchphrase for the show."
Was it originally in the script or was it something you added?
"It was added to the script during rehearsals. I believe that a producer added it during one of the last-minute rewrites we have right before we did a live taping.
"Somebody decided that we needed to add a word to the script, and then they said, 'What about 'bazinga'?' It's really funny how these things happen. I said it that day, and it's stuck ever since."
What do you like the most about Sheldon?
"I feel like Sheldon seems to approach every situation by trying to figure out the scientific nature of things. I like how his scientific reasoning so frequently doesn't work in the real world when he's dealing with Penny or when he's out in a restaurant - but he sticks to his guns. I really like that about him. He's very sure of himself in a lot of ways and I think that's very neat."
What have you learned about science and technology since working on the show?
"Scaredly, I've learned very little. It's a shame, but every time I get something scientific in the script, I read up to find out what I'm talking about - but then I'm on to the next script and it's forgotten. There's no absorption of information. I'm no smarter since appearing on the show. In fact, I might be a little dumber because I haven't had time to learn anything else."
What's it like to work alongside former Blossom child star Mayim Bialik?
"It's fantastic. She is amazing. I can't say enough good things about her."
Didn't she leave the entertainment industry years ago to become a scientist?
"She did. She became a neuro-something or other, which means she works with the brain. It's amazing. We had an episode last season where she had to slice a brain and she had this huge discussion with the producers about how thinly you would need to slice it. I remember thinking, 'Ouch!' This is unreal."
Do you think geeks can be sexy?
"Oh, yes. Definitely. Especially along the lines of intelligence. I think intelligence is usually sexy until it becomes irritating. After that, you're stuck."
New episodes of The Big Bang Theory continue on Thursdays at 8/7c on CBS. It airs on E4 in the UK.