There was a palpably celebratory air on the set of “The Big Bang Theory” as the cast and crew shot the show’s upcoming episode “The Celebration Experimentation.”

To mark Sheldon Cooper’s latest birthday, the series regulars – Jim Parsons, Johnny Galecki, Kaley Cuoco, Mayim Bialik, Kunal Nayyar, Simon Helberg and Melissa Rauch – were dressed in their characters’ classiest wardrobe. Batman-themed balloons and decorations filled Sheldon’s geeky tchotchke-cluttered apartment. Special guest stars – including perennial favorite Wil Wheaton; Christine Baranski as Leonard Hofstadter’s mother; Sara Gilbert as Leonard’s ex-paramour Leslie Winkle; John Ross Bowie as Sheldon’s scientific rival Barry Kripke; Kevin Sussman as comic shop owner Stuart Bloom; and even TV’s first Batman himself, Adam West – mingled merrily with the leads.

But it was an off-camera touch that revealed the extra sense of accomplishment creator Chuck Lorre’s team was feeling; three sets of silver Mylar balloons tied to the railing where the live audience would sit on the night of the taping in the shape of the numeral “200” – as in 200 episodes over the top-rated show’s nine-season run to date.

“It’s very funny being here this week,” said Parsons, decked out in a celebratory suit far nattier than Sheldon’s typical superhero logo tees, “because not only are there balloons for the party that’s happening on set, there’s balloons for us around here, there’s press around here, and so in that way, you can’t ignore it. It’s like being in the New Hampshire primary.”

But Parsons, the recipient of four Primetime Emmy Awards for his performance throughout the course of the series, was pleased that content-wise, the 200th offers generally funny-business-as-usual.

“It’s an interesting thing doing these episodes that note a number of episodes because they tend to always fall somewhere midseason or beyond that at this point,” he said. “It’s not like a finale, a season opener, there’s no cliffhanger – and that I like about it. I like just another day at the office as often as possible. I always try to take my foot off the gas about anything special. Even when Sheldon and Amy had sex this season, it was like, ‘How to take the momentum off of this?’ actually.”

“It’s good that it feels like another day at the office,” agreed Bialik, herself oft-Emmy-nominated for her role as Sheldon’s paramour Amy Farrah Fowler. “It doesn’t feel like we’re a show that has some problems with its writing, and we’re dwindling in our talents of what we’re producing, so let’s do a big episode to get lots of press. It feels just like a normal week for us that happens to be 200 – and I think that’s a good place to be in season nine, right?”

Still, Galecki admitted to holding back some emotional waterworks behind Leonard Hofstadter’s glasses. “It’s surreal – I’ve been really hypersensitive and sentimental about it all week,” he said of the landmark. “I mean, there’s hundreds of ingredients and not a single recipe. There’s not a manual for this. So even with a common goal in mind and with dedication and commitment at its deepest and with an incredible pedigree of talent gathered, you just have no idea.”

He added, “You don’t know if it’s going to work or not. It often doesn’t. We know this. It more often than not does not work. So I’m incredibly proud and humbled and honored to be here.”

“It’s a huge deal,” agreed Cuoco, admitting that early on she would not have imagined she’d walk for nine seasons in the shoes of Penny (whose maiden name has still never been uttered). “I don’t think anybody would ever think you’re going to make it this far. We knew we had something special during the pilot, but this has been icing on this huge cake now for many, many years. So we feel very lucky – I just can’t believe people are still watching the show. It still surprises me.”

“As an actor, you dream about stuff like this,” said Nayyar, who was ready to return to India for a career in Bollywood films before he landed the role of Raj Koothrappali. “You see it happen to other people, and you dream about it. And sometimes you think it won’t happen to you – and it does. And you get to live it. And speaking for myself, dreams do come true, and I’ve dreamed about this for a long time. And I’m lucky to be living it. Very, very lucky to be living it.”

“I don’t even know that I knew to dream of this kind of thing,” added Helberg, who recalled that another acting gig nearly derailed his offer to join the cast of the series as Howard Wolowitz when it premiered in 2007. “To be with such an incredible group of people, telling a story that you like to tell, playing a character that you love to play and getting to grow with those characters over what will be ten years is so rare.”

And in the show’s long and storied tradition of recruiting geek culture icons to guest on the series, the producers saved one big Bat-get for the milestone episode.

“It’s a great tribute – it’s wonderful,” said West. “For years I thought ‘Oh, they’re just ignoring me. They don’t want me around.’ And then I found out that they were saving me for the 200th anniversary show – knowing, I think, too, that it’s ‘Batman’s’ 50th anniversary – our show – since it went on the air.” West, of course, plays a comic spin on himself, “who likes to insult some of the people who’ve done Batman more recently. I get a chance to insult a lot of big stars!”

The actors acknowledged that while they’re inching closer to still-unplanned end of the series, they tend to avoid discussing or even contemplating the day they bring it to a close.

“I don’t want to think about it,” said Rauch, whose character Bernadette Rotenkowski-Wolowiz joined the show in its third season. “There was one day that we were kind of talking about it here – like, what that would be like? and all of us were very sad and changed the subject.”

“Yeah, we kind of don’t talk about the end,” said Cuoco. “All of us are in the same boat. I think we all want to be here as long as we possibly can. I don’t see the end any time soon.”

“The biggest common denominator of why life is still so joyful here is because we enjoy doing the stories every week and playing those characters,” added Parsons. “It’s nine years. I can only imagine other circumstances where you’re just like, ‘I wouldn’t dare leave this job, but oh my God, I have to go back?’ That’s just completely the opposite here.”

“The Big Bang Theory” 200th episode airs February 25 at 8 p.m. on CBS.