This Is Us: Justin Hartley Breaks Down Kevin's 'Impulsive, Sometimes Catastrophic Decisions'

From a spur-of-the-moment marriage to blossoming love and familial gatherings, This Is Us dove into the complexities of relationships in the Milo-Ventimiglia-directed “Storybook Love” episode.

Audiences learned more about Kevin’s marriage to Sophie, which was a spontaneous decision in New York — much to the surprise of his mother Rebecca and siblings Kate and Randall. Fans already know that the marriage doesn’t last, and Justin Hartley tells PEOPLE that his character’s “fear of being alone” will continue to drive his ongoing state of being a hopeless romantic.

“His fear of being alone creates these impulsive, sometimes catastrophic decisions just based solely on that,” the actor, 42, explains.

The tragic and unexpected death of Jack resulted in the Pearson family members spinning in different directions of grief, with Kevin desperately seeking stability by making the rash, romantic (and life-changing) decision to marry.

“Impulsive is a great word,” Hartley says of his character’s state of mind. “He’s got a romantic view on things, and I think sometimes it’s very, very appealing. And I think sometimes it’s what is super charming about him at certain times. But also I think it’s what gets him into trouble, because he doesn’t often think about the unintended consequences of these big grand gestures that he often does. Like marrying his high school sweetheart or like moving to L.A., moving to New York, and moving in next to his uncle.”

In the present day, Kevin’s love life has stalled since he broke it off with Zoey (Melanie Liburd) last season, so he has a renewed focus on his sobriety. However, he’s been masking his own issues by concentrating on the wellbeing of uncle Nicky and military vet Cassidy, whose marriage Kevin volunteered to help save.

“I love Kevin’s commitment to helping Cassidy. I don’t think she has anyone. She was gone and came back, everything’s changed. And I can only imagine when you’re starting over as an adult, it’s not a natural thing,” Hartley shares. “For Kevin to realize, ‘I’m going to be her friend, I’m going to take some s— from her. I’m going to help her get her family back and I’ll help her fix this whole thing,’ it’s [acting] like nothing is broken.”

Fans have nicknamed Kevin, Cassidy and Nicky the “new Big Three,” but Hartley refers to them as “three lost souls.”

“Whenever you develop a friendship like that, you have kindred spirits. There are things that those people have in common. And one of them is this huge [alcohol] addiction,” he explains, adding, “Misery loves company.”

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Hartley continues, “They’re all losing something, and they’re all trying to find something that they’re not sure that they should even have or that they even deserve. So in a way, you could say they deserve each other. When it seems like they’re absolutely so down on themselves and they’re not feeling too good about who they are, I think you’re going to find that like, ‘Thank God they have the other ones.’ “

As for what’s to come, Hartley hints at Kevin and Nicky growing even closer (possibly over more bricks of ice cream).

“It’s going to be an interesting thing to see,” he teases. “Obviously Kevin’s moved in next to Nicky, he’s physically there next to him. That relationship is going to evolve. Maybe you could say that they’re new best friends or something.”

Speaking of best friends, Hartley was finally able to work with longtime cast member Ventimiglia on TIU, though it wasn’t exactly on-camera.

“Because of the way that the story works on the show, it doesn’t make a lot of sense for us to be working together, so it was nice to actually be at work and have him there even in that capacity. It’s cool, it’s awesome,” Hartley says of his TV dad making his directorial debut. “He’s my buddy, so it’s nice to have him, and he’s a very, very capable, very smart guy. I don’t think anyone was worried about how he was going to do. Everybody knew he was going to be great at that. It was just nice having him there. And I like Milo, I like hanging out with him, I like being around him.”

And Hartley will be following in his costar’s footsteps. “I’m going to direct later in the year,” he says.

This Is Us airs Tuesdays (9 p.m. ET) on NBC.