'That '90s Show's Debra Jo Rupp, Kurtwood Smith on Working With Mila Kunis, Ashton Kutcher Again (Exclusive)

Smith and Rupp opened up to ET at the show's premiere in Los Angeles.

For That '90s Show, the original cast of That '70s Show reunited for a whole new look at the lives of their beloved characters. For Kurtwood Smith and Debra Jo Rupp, one of the most unexpected aspects was working with Mila Kunis and Ashton Kutcher as a now real-life couple.

ET's Denny Directo spoke with the stars at the premiere in Los Angeles, and the pair opened up about getting everybody back together for the new series, and stepping back onto the old set."

"I think [the same thing] happened with everybody on the old cast -- we call them the legacy cast -- it's just like a lot of photos, a lot of memories," shared Rupp. "So many memories (all at once), it was almost overwhelming."

For Smith, returning to the Forman house "was just like going home."

As for the fact that Kutcher and Kunis went from an on-screen romance to a real-life marriage with two children, Smith laughed, "It's wild."

"I mean, they are the most beautiful couple I have ever seen in my life," Rupp said. "They are just gorgeous, lovely people. Beautiful people."

"I couldn't have predicted it, but at the same time it didn't surprise me," Smith said of their adorable romance. "I was like, 'Oh, OK, yeah. That makes sense."

That '70s Show went off the air in May 2006, and after all those years, Rupp explained that it took some time for Kunis to recapture the voice and mannerisms of her character, Jackie Burkhart.

"Mila had to find Jackie again," Rupp recalled. It's so far away from who she is now, so it took her a couple of runs there to get back into the Jackie mode."

The same couldn't be said for Rupp and Smith, who both agreed that they fell back into their characters almost instantly.

"When you have really good writers, like we do, they know how the characters speak," Smith shared. "So it's pretty easy to slide back into it."

That '90s Show premieres Jan. 19 on Netflix.