This Is Us : Griffin Dunne Talks Nicky's 'Complicated' Past and How He'll 'Overstay His Welcome'

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Uncle Nicky is back on This Is Us — and he may be sticking around for a while.

As fans can recall, Jack (Milo Ventimiglia) previously told his wife Rebecca (Mandy Moore) and kids that Nicky had died during the war. Then in season 3, Kevin (Justin Hartley) discovered a post-war postcard from Nicky to Jack and the Big Three discovered that their uncle, who was struggling with alcoholism and PTSD, was in fact alive and living in an old trailer in Bradford, Pennsylvania, where he still lives. Fast forward to season 5, Kevin named his son Nicholas after his beloved uncle.

And Tuesday's episode, titled "One Small Step," gave more backstory into the life of Nicky Pearson (Griffin Dunne), who, in the present-day, surprised nephew Kevin and his fiancée Madison (Caitlin Thompson) at their doorstep in Los Angeles after receiving an invitation to their twins Nicholas and Frances' baptism. Though it was planned to take place on Zoom, the Vietnam veteran didn't realize that and got on his first plane ride since the war instead.

While Nicky downplayed to Kevin just how big of a deal it was for him to fly all the way to L.A., audiences learned more about why the cross-country flight was so major and what happened in his past, including his first love Sally (Genevieve Angelson) as well as his estrangement from brother Jack.

RELATED: This Is Us: Why Jack Kept Brother Nicky a Secret — and How It 'Profoundly Affected' Him

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Before he was drafted into the war, Nicky, who Michael Angarano portrays in the younger years, and Jack joined their parents Stanley (Peter Onorati) and Marilyn (Laura Niemi) to watch astronauts Buzz Aldrin and Neil Armstrong's historic walk on the moon in July 1969. At the time, the brothers were closer than ever and Jack helped Nicky navigate his first romance with Sally, a photographer and vet clinic coworker.

But after choosing to stay with his parents instead of running away to Woodstock with Sally, Nicky was stuck in Pittsburgh and eventually shipped off to Vietnam, where traumatic events led to a falling out with Jack, whom Nicky had a missed opportunity to reunite with once back home.

Below, Dunne, 65, tells PEOPLE all about his character's past and just how long Uncle Nicky might be crashing at Kevin and Madison's house on the west coast.

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PEOPLE: So sad about those snow globes for the twins! They illustrated two important stories from Nicky's life. But the John Grisham novels will do I guess, especially with that smiling photo of Jack and Nicky.

GRIFFIN DUNNE: It was a long journey, but they get the Grisham books so it all sort of worked out.

Cassidy (Jennifer Morrison) is one of few friends Nicky can count on, but now that Nicky is in L.A., will trust be an issue for him or will he be fine because he's with family?

Well, nothing is ever quite so simple with Nicky. He's an overthinker and highly neurotic, and I think he was like that before he went to the war. As we see with Angarano's Nicky turning down an opportunity that could've been wonderful, he turned down a great love affair and going to Woodstock to see Hendrix. All that self-doubt, overthinking, neurosis, and self-loathing, as Cassidy says Nicky has, I think he's always going to be grappling with that.

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While Nicky was dealing with self-doubt, when he was with Jack he received self-assurance. Can you talk about that brother relationship and why you think it changed so much during and after the war?

Jack was much more suited to handle the horrors of what happened in Vietnam. There's a dynamic in every family and one sibling is just going to be more sensitive than the other. That was certainly in Nicky's case in relation to Jack. The sadness is that the sensitive one is the one who got drafted and couldn't be less suited for the horrors that he would be seeing. That led him to the most traumatic event of his life in killing the little boy accidentally in Vietnam.

I think he had been in such a self-destructive spiral and his brother was trying to help him so much, in such a way that sometimes you could help someone who is troubled and needs help. But you could also do it in such a way where the one who is troubled just feels so condescended to and it reinforces their own self-loathing. And [the latter] was the dynamic, it makes Jack over persistent and overprotective. It emasculates his brother unintentionally. It's just a sad dynamic that happens to two people that love each other. It becomes too draining on both of them. So Jack deciding to let go of his brother by telling the story that he died, it's a sort of survival for Jack but it leaves a ghost and friction between the two of them.

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In season 3, we saw that Jack showed up at Nicky's trailer and told him about his wife and kids, even revealing a happy family photo. Nicky has spent Thanksgiving with the family and now has met his great-niece and great-nephew. Do you think Jack would've been proud of who Nicky is in the present-day?

Not only he'd be proud, but he would also be envious of his son and proud of his son that he got to have a relationship with his brother. He'd be enormously touched and proud of the effort that Nicky has made to be part of Jack's family.

The moon metaphor was a great way to tell how far Nicky has come. Especially this monologue: "July 20, 1969, I watched a man walk on the moon. One day we'd never been on the moon, it was impossible to even fathom walking on it. And the next day, we're walking on it. The impossible became possible, just like that. For 50 years, I lived in a trailer. ... And I got that invitation in the mail but an invitation to meet my great-nephew and my great-niece. My big brother's grandchildren. I've lived alone for a really long time. I've been stuck. Anything good that would ever happen to me, just seemed impossible, but here I am, I made it with you two. You two are my moon."

I thought it was extraordinarily good writing. I read it just as a curious reader of how they made that metaphor work in terms of the impossible becoming possible. I found it relating the moon landing and Nicky's small step toward family, which was enormous for him for crossing the country. All those elements were very clever and emotionally satisfying. It didn't force it, it was something that took you by surprise. I was quite moved by it.

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Has Nicky hit a turning point in his life with him meeting his namesake?

I always say [to the This Is Us staff], not necessarily joking, "Please don't let Nicky get his life together too well." I like playing the complicated aspects of him and troubled aspects. I would hate for him to get incredibly healthy all of a sudden. It would be less for me as an actor to play.

In a previous chat with director Ken Olin, he said, "You'll see that when Nicky arrives, Kevin's very safe haven with Madison is going to have some ripples there." How long will Nicky's visit turn out to be?

There's some expression about that: A guest can actually start to go bad after a while in the fridge. I think he's going to overstay his welcome. He's a difficult guy. I channel my father a bit, I channel Frances McDormand. She's so good at being irritable and difficult on the screen. I remember my father coming to stay with me and my daughter, and because of his age and particular quirks, everything revolved around him in the time that he's staying. He has to have things just so and I have a feeling Nicky's going to be a guy like that. His eccentricities will take center stage.

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But babies can change people.

Babies can change people. Again, with my dad, I never saw him light up so much as when my daughter was born. He was excited as I have ever seen him. And I had the same reaction as most people do when they see their fathers become grandparents. They just love to spoil and you see a side ... and it's just easier to be a grandparent than the parent. I think Nicky will be soft towards the kids but I still think he will be grappling with the adults around him.

In a previous flashforward, the Pearson family, including Nicky, is gathered by Rebecca's hospice bedside at the house Kevin built with Jack's architectural designs. Knowing that Nicky is in her room and appears to be a mainstay in the family, what can you tease about the close bond he forms with the rest of the family?

I know as much as you do and I had the same observation you did. I don't know how he got there but he seemed to be, sort of, the patriarch. Nicky has a wedding ring that they cut to, and I wish I could even tell you, "Oh I know who he married and I can't tell you," but I can't tell you because I don't know who I'm married to.

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