One of the stars of that show, Mandy Moore, is reportedly engaged to her boyfriend of two years, indie rock band Dawes frontman Taylor Goldsmith.
"I was just in the studio with [Moore's] boyfriend, well now fiancé, Taylor," said Sullivan, who plays Toby on the show. "He didn't say a word. He seemed a little effervescent, but he's always pretty effervescent."
Sullivan said he had yet to get all the "How did he pop the question" details from Moore, but said he had seen her engagement ring.
"She just sent out an alert over our family text chain, so I can't wait to hear about it," he said.
Moore, who was previously married to musician Ryan Adams for seven years, celebrated her two-year anniversary with Goldsmith in July.
"The best 2 years," she wrote on an Instagram picture of them together. "Never not smiling with you, T."
Jon Huertas, who plays Miguel on "This Is Us," told "ET" that Goldsmith is "an amazing guy."
"I love him so much. I love Mandy so much," Huertas said. "I'm really happy that they're going to be joining the club."
As of Thursday, neither Moore nor Goldsmith had spoken publicly about an engagement.
But the actress had good things to say about her beau over the weekend, during her keynote address at the Create & Cultivate
"I think a lot can be attributed to I love my life right now," Moore said. "I'm in a wonderful relationship, and I have a job that I love, and I'm so challenged by and satisfied with [it.] That kind of makes all the difference."
CNN has reached out reps for Moore for comment.