Milo Ventimiglia and Mandy Moore "almost took vows" before 'This Is Us' premiered.
The 44-year-old actor plays Mandy's on-screen husband in the TV drama series, and Milo has promised that they will always support each other.
He said: "Right before the show premiered, we were in New York for the upfronts ... and everybody was kind of milling about and Mandy and I were just kind of off to the side by ourselves and we just kind of raised our glasses to one another.
"We almost took vows with one another where it was like, 'Hey, I promise to always communicate. I promise to not be happy until you're happy. I will always look out for you.' And we have stayed very true to what we said in the beginning."
Milo also described Mandy, 37, as the "best" on-screen partner he's ever had.
He praised the actress' "commitment" and "dedication" to her work.
Milo said on 'The Tonight Show Starring Jimmy Fallon': "I've got to say she has been probably in my 26 years in front of the camera, the best partner I've ever had - her commitment, her dedication, her growth as an artist.
"The audience, you guys get to watch the show in its entirety. I get to sit there in front of her, or stand in front of her, or lay in a bed next to her and watch front row what she does and how impactful it is, the work is to her. And I've only seen it grown. So, it's been a pleasure to be her TV husband for six years.
"At the same time, closing this chapter, I'm very excited to see what Mandy does next because she's such a force."