One of the most surprising revelations from the season-five finale of "This Is Us" that aired last May was a flash-forward scene that left no doubt about the upcoming breakup of Kate (Chrissy Metz) and Toby (Chris Sullivan).

In the scene, Kate appears in a wedding dress and her boss Phillip (Chris Geere) addresses Kate's brother Kevin (Justin Hartley) as his “future brother-in-law."

Though Kate and Toby's marriage has had its share of problems, learning that the couple ultimately parts ways caused many fans to grieve. But in a new interview with Entertainment Weekly, Sullivan revealed that he and Metz have known about the split for "a couple of years" — and promised that the hit NBC drama will handle it "delicately" in its upcoming sixth and final season.

"We didn’t know how or when it might all occur. But we’ve known for a while. And of course, as soon as (series creator) Dan (Fogelman) told Chrissy and I, it was very sad," he said.

The two-time Emmy nominee said it's been emotional for him to play one half of a beloved couple, knowing that the couple will eventually call it quits.

"That’s tricky information to have when you’re just rolling along, enjoying this romantic comedy of a relationship," he said.

"To know that eventually it’s going to dissolve is hard to handle. But if 'This Is Us' is a handbook on relationships and exploring every aspect of a relationship, somebody has got to split up — and it wasn’t going to be Randall and Beth," he continued, laughing.

For her part, Metz previously told Entertainment Weekly that she broke down crying when she learned about Toby and Kate's fate — and even tried talking the show's writers out of the idea.

Sullivan told EW that he and Metz have largely avoided talking about the breakup until recently.

"We’ve had to talk about it a little more toward the end of the season, but we don’t really talk (about it)," said the actor. "I mean, there’s nothing really to talk about. It’s not a fun thing to talk about."

Sullivan said that he and Metz eventually came to a "somber acceptance" of what Fogelman and the show's writers were trying to do with the storyline. He added that, as actors, it will be "an interesting challenge" for him and Metz.

But as for the details about the breakup, Sullivan still has no clue.

"I don’t know how the eventuality of it all is going to play out. If you’re trying to ask me whose fault it is, I actually don’t know. I’m glad I don’t know," he said, laughing.