Hartley says Kevin's struggles have inspired people to "reach out" to him.
"It touches people," Hartley said of Kevin's story. "People walk up to me all the time telling me intimate, personal stories — strangers!"
He also discussed Kevin's story with his friend and recovery activist Ryan Leaf. "He went through a similar addiction and he has come out on the other side and ... now ... he speaks to schools and he travels the country and he's an amazing human being."
Hartley attributes the show's exceptional relatability to the writing team. "It all starts with the writers.... It's kind of like when ... your pitcher is throwing a no-hitter."
"You don't look at him, talk to him, act like they're not there," Hartley said. "You just sort of let them be in their own air and that's what they're doing right now. They're incredible."
Hartley also spoke about whether he believes Kevin will continue on his downward spiral after an emotional and poignant low point in last week's episode.
Hartley hopes that Kevin's story will caution viewers from following the same path. "One of the things you hope for is that if there's one person that can reach out and get help -- then we've done our job."
He talked marrying actress Chrishell Stause late last month after dating for four years.
Many of his "This Is Us" co-stars joined him for the reception, including Sterling K. Brown, who plays Randall Pearson.
Hartley said Brown caught the garter -- with Brown's wife, Ryan Michelle Bathe, and on-screen wife, Susan Kelechi Watson, both present!
"You know what he is? He's ... very competitive," Hartley said. "It's one of the things I love about him ... when he got up and walked to the dance floor, I was like, 'He's going to catch it!'"
Hartley also weighed in on him and his "This Is Us" co-stars being named "Sexiest Cast" by People magazine.
"I don't know but I got a lot of help from my two co-stars there, those are two good-looking dudes," Hartley said of Milo Ventimiglia, who plays Jack Pearson, and Brown. Hartley said the sexiest "would be between Milo and Sterling, for sure" — he joked: "This guy got third place!"
Hartley also shared sweet words about his 13-year-old daughter: "She thinks I'm pretty cool. She's the highlight of my life."
"It's awesome," he continued. "She's honest. We talk to each other like on this level, and she's kind and she's sweet. She's forgiving. She's generous ... I blame the parents!"