Haley Lu Richardson on How She Proposed to Her Boyfriend,' Jane the Virgin' Star Brett Dier

Haley Lu Richardson is opening up about getting engaged to her fiancé, Brett Dier.

While appearing on Monday night's episode of Busy Tonight along with her Five Feet Apart costar Cole Sprouse, Richardson, 24, told host Busy Philipps, "I asked him to marry me!"

"I just asked," the actress continued. "Neither of us are like crazy romantic people, so nothing was planned. He was sitting there eating a pizza and he was telling me a joke that Shia LaBeouf had said in an interview, and he was like pretending to be Shia LaBeouf explaining this joke and for some reason I just cut him off and was like, 'Wanna marry me?'"

"You married Shia LaBeouf" Sprouse, 26, quipped.

"Maybe that got us in the mood, I don't know," Richardson laughed. "But yeah, we're engaged now."

Richardson and Dier, 29, had been dating for years before she popped the question. In an interview with Cosmopolitan in February, the actress said that they hadn't set a wedding date yet.

"We've been together for seven years," Richardson told the outlet. "We just want to take our time. We're still not used to the word fiancé. It's so pretentious in saying it. Neither of us are used to it. We still call each other boyfriend and girlfriend. We're just engaged. And I love Brett. And we'll get married one day."

"I had no idea when I woke up that morning that I was going to ask someone to marry me that day," Richardson added to Cosmo. "I was totally in the moment and this gut feeling came up and coerced me to ask him to marry me and I did. And he said yes. I feel like the trick to that was just not thinking about it at all before it happened. I just kind of did it."

Richardson also revealed that they used a surprising material for their on-the-spot engagement rings: twigs!

"We were at this outdoor strip mall eating pizza when it happened," she told Cosmo. "When we were crying and having this really amazing moment that I'll never forget, crying and holding each other and saying oh my gosh, we're going to get married, we're going to be together forever, we were like, we're gonna have to do something about it because we didn't have any rings."

"So we went over to this bush and got these twigs and each knelt down one at a time and tied these twigs around our ring fingers," the actress continued. "We have them saved in a little baggie. My engagement ring now, it's custom made, it's a really simple ring but it's engraved like a twig."

Five Feet Apart is in theaters now. Busy Tonight airs Mondays through Thursdays at 11 p.m ET on E!