Inside the 'Big Bang Theory' Wedding for Penny and Leonard

Everything you need to know about the upcoming nuptials.

— -- The season 9 premiere of "The Big Bang Theory" is a "big" one.

It opens in Vegas with Leonard and Penny getting married, played by longtime "Big Bang" stars Johnny Galecki and Kaley Cuoco-Sweeting.

Showrunner Steve Molaro spoke to the Hollywood Reporter and dished on what to expect.

"It's season nine and Leonard has been pursuing her since minute three of the series," he told the magazine. "Even Penny, with all of her commitment issues, is in a mindset of, 'Are we moving forward with this or not? What are we doing?' And they do."

But one thing might beholding the couple back. Leonard kissed another woman a short time back.

"She thinks she's OK with it and she may not be," Molaro continued. "It was a few years ago, but it fluttered. It's the kind of thing that you wouldn't expect to happen and that can be a little jarring. I think when you're Penny, and you have an image that you've cultivated of a person for eight years now and those two things aren't aligning anymore."

He added that Leonard will have t speak to this other woman to "put this fire out."

"Things will start to get better, not necessarily because of that, but Leonard and Penny are both invested in figuring out how to fix this," he said.

In the season, we will see Penny's father Keith Carradine and how he reacts to them eloping.

Even though they eloped in Vegas, Molaro said it's still a beautiful episode.

"There's a really special moment, despite all of the things that are happening, that I think makes Leonard and Penny -- and especially their vows -- truly a magical moment that I'm really excited about," he said. "Johnny and Kaley just killed it, it's so good. It was one of those things where I even called them before we were back [in production] just to tell them how excited I was about it."

Then there's Sheldon.

"Living arrangements are definitely going to be confronted in the first four episodes. It's tricky, sure -- Leonard and Penny need to move forward, but at the same time we have a broken-hearted Sheldon (Jim Parsons), who is sort of their little brother and child and it's tough to ignore that for them," he added.