Mila Kunis on How She Fell for Ashton Kutcher

The former co-stars married last year and are expecting their second child.

"I think it took him and I both by surprise," she told Stern on his SiriusXM show. "We started dating with the idea that we both were never going to get married."

The leap to even dating was big enough. Although Kunis had known Kutcher since she was 15 when she was cast alongside him on "That '70s Show," there were never any sparks between them.

"I see this guy and I see his back and he's very tall," Kunis told Stern, adding that she looked ridiculous in a "big poofy dress."

The guy turned around and it was Kutcher.

"It was literally like if we were in a movie, the music would start playing," Kunis recalled. "I think he, for the first time ever, literally took my breath away."

Although they had kept in touch over the years, they spent the evening catching up and before they parted, Kutcher invited Kunis to his housewarming party.

Nervous for the first time, Kunis brought along a girlfriend "just in case," and unbeknownst to her, Kutcher had invited a friend he had planned to set her up with.

But after Kunis arrived, he didn't bother making the introductions. Instead, Kutcher and Kunis shared their first romantic kiss together, and Kunis spent the night at his house.

Still, the pair weren't ready to commit to each other.

"We're both in agreement that this is just fun, shook hands on it," she said. "Three months later, I'm like this isn't fun anymore."

That's when Kunis realized she was falling for him. And even though Kunis had been saying since she was 16 that she would never get married, she said yes when Kutcher popped the question in February 2014. They were married the following year after she had given birth to their daughter Wyatt Isabelle.

"I think we both had to go through a lot in our lives to become the people that we were," Kunis said. "We would never be together based on the people that we used to be."