'This Is Us' star Milo Ventimiglia said he 'took vows' with costar Mandy Moore before the series premiered and called her 'the best partner I've ever had'

• Milo Ventimiglia called Mandy Moore "the best partner I've ever had."
• He told Jimmy Fallon that before "This Is Us" premiered, he "took vows" with his costar.

"This Is Us" star Milo Ventimiglia discussed on Tuesday's episode of "The Tonight Show With Jimmy Fallon" just how real his on-screen marriage to Mandy Moore actually is.

He said that at an NBC upfronts event in New York before the series premiered, he and Moore found themselves alone in a quiet corner. They toasted each other and the new journey they were about to go on.

Ventimiglia said the pair "almost took vows with one another."

Some of the commitments they made to each other, according to Ventimiglia, included: "I promise to always communicate. I promise to not be happy until you're happy. I will always look out for you."

The costars don't seem to have taken those vows lightly. "And we have stayed very true to what we said in the beginning," Ventimiglia added, calling Moore "the best partner I've ever had."

Moore and Ventimiglia play married couple Jack and Rebecca Pearson on "This Is Us," and though the characters' three children are the center of the present-day story, Jack and Rebecca's relationship is at the show's heart no matter what decade audiences are watching.

He previously told E! that he and Moore "both agreed that from action to cut, for all intents and purposes, we're a married couple."

Moore, who is married to Taylor Goldsmith in real life, previously told Popsugar that she felt natural chemistry with Ventimiglia almost instantly. She said he asked her if it was okay that he "get close" to her and when she agreed, "he nuzzled into my neck and pretended to be my husband."

"I was like, 'Oh, wow, this is how it's supposed to feel.' It was great," she added.

"This Is Us" airs Tuesdays at 9 p.m. ET on NBC.