"He's just f—ing with me," the Kevin Garvey actor tells THR about the biggest laugh of the series thus far, as written by Damon Lindelof.

[Warning: This story contains spoilers for the penultimate episode of HBO's The Leftovers, "The Most Powerful Man in the World (and His Identical Twin Brother)."]

It's not always doom and gloom in the world of The Leftovers, despite what events like the Sudden Departure and the equally sudden departure of figures like Laurie Garvey (Amy Brenneman) would have you believe.

Case in point: "The Most Powerful Man in the World (and His Identical Twin Brother)," the HBO drama's penultimate episode, written by Damon Lindelof and Nick Cuse. Within the two-hour mark of the series finale, Lindelof and Cuse were able to deliver what was quite easily the biggest laugh in Leftovers history: Kevin Garvey (Justin Theroux) somehow holding office as President of the United States in the underworld of sorts first introduced in last season's "International Assassin," needing to confirm his identity in order to access a secret bunker. The two-step process involves confirming what's described as Kevin's "unique biometrics." Part one is easy enough, as Kevin is scanned for facial recognition. The second part? He must place his penis on a scanner.

"Mr. President, due to advances in plastic surgery, anyone with enough resources can copy a person's face," says Kevin's bodyguard, the deceased Australian police officer also named Kevin (Damien Garvey). "But your penis, sir? Well, they're not going to go to that length."

The gag lands (with a thud, it's worth noting) on its own, but it provides an extra punch for behind-the-scenes reasons: Justin Theroux made headlines during the first season of The Leftovers, when a jogging scene in an early episode showed a little bit more of the actor than he bargained for. Unfortunately for Theroux, Damon Lindelof has an enduring twisted sense of humor. It's fortunate for the viewers, however, as it provides a much needed moment of levity, especially in the wake of the show's most recent episode, in which Kevin's ex-wife Laurie commits suicide.

Lindelof's interest in Kevin Garvey's biometrics extends to the man's heart, quite literally based on the end of the episode. In "The Most Powerful Man in the World," Kevin once again wanders through an other-worldly realm in order to provide answers for the folks who believe in him — people like Kevin Senior (Scott Glenn) and John Murphy (Kevin Carroll). Things take a turn when Kevin realizes he's in two places at once: he's both an assassin in this world and also the president, playing the dual role of twin brothers at opposite ends. With the help of another ghost from the past, Patti Levin (Ann Dowd), Kevin literally confronts himself and reaches the conclusion that he wrecked his relationship with Nora Durst (Carrie Coon). With that, he reaches his hand into his double's chest and fixes his own heart, subsequently deciding to nuke this strange world and return to the one that truly matters.

For more on Kevin's return to the "International Assassin" world and the powerful epiphany that came with it, read on for what Theroux told The Hollywood Reporter about the episode — and yes, that includes a quick sojourn into the "unique biometrics" of it all.

Heading into season three, it felt like there was an unspoken promise that we would return to The Leftovers' version of the underworld — whatever this absurd place is — thanks in large to its reappearance in season two's finale. Did you feel the same way, that it was predestined we would come back here?

I don't know if it was predestined. I know Damon was mulling it over in his head, but that he had the same reservations that I had: we don't want to paint the same barn the same color. Let's not just do it because people like it. If we can think of a good reason to do it and a good reason to go back there, let's do it for sure. I think he achieved that, which is obviously that there was this question of whether Kevin is killable, this messianic through line that's happening, and there's a reason for him to go back there. He can service all of these people and answer some of their questions. The thing I thought was a twist that made it worth going back there ultimately was that it ends up being a story about the completion of Kevin, and literally cutting this thing out of him and sort of killing his shadow self. Brilliantly, doing it with the assistance of Patti, who he was able to help in the initial "International Assassin" episode.

She came back to return the favor.

Yeah. She gets to come back and help me. Which I thought was a beautiful piece of storytelling. In that sense, it feels very different.

Which Kevin died on that day?

I think he killed the Kevin that's in his real life, the Kevin that's constantly running away and slamming doors on Nora, and trying or teasing to go to the other side by putting plastic bags on his head — whatever that is. He's literally cutting out his own disfunction, and basically owning his shit for the first time, in a way.

There's a moment when President Harvey says: "This isn't my first time visiting the other side of the world. Every time I'm here, it gets harder and harder to leave." We've seen Kevin wearing bags over his head this season. We see him in this episode plunging himself straight into the water, not waiting for his so-called apostles to wake up and make sure he can come back. Does he not care if he doesn't come back? Is this his preferred world? To borrow a phrase from the show: does he want to live here now?

Yes, I think he wants to live there now — well, not there. I think he wants to live above ground. But I think he needs to discover that and find that and earn that. Even Patti pulls him to the mat and calls him out: "What do you want?" "I want to go home." "You keep saying that, but you're constantly leaving home. You're constantly uprooting yourself and going places and walking out on your family and doing insane things." Again, in that quote-unquote "romance novel" he's writing, when she finally forces him to read his own words, it's this love story ostensibly about a guy who gets on a boat and sails into the middle of the ocean alone. That's the moment Kevin realizes that that's the way he's designed his life throughout. When things get difficult, he always leaves a back door for himself, in terms of self preservation. It's a selfish act. She's the one who really calls him to the floor and pins him on it. That's why he has the reaction, and why the other Kevin says, "Cut this thing out of me." I think he wants some sort of self-provided absolution.

The whole episode builds to that moment, when Kevin tells himself: "We f—ked up with Nora." There's the reading of the love story, the use of "God Only Knows," as Kevin fixes his heart. Does this express a point that for a show that can be saturated with so much grief and very depressing subject matter, is it also a love story in a way? Is it a romance in the end of things?

I think so. You know, it's obviously not a traditional love story. It's not The Notebook of Kevin. (Laughs.) Just to reach back and tap Damon for a second, he says this is a family drama. It's about family. It's not Parenthood and it's not Family Ties, but it is about family, and romance is a part of that. The love story of Kevin and Nora is central. It's obviously the two people arguably the most dysfunctional, and the two people who keep orbiting and smacking into each other and hurting each other. In that sense, it is kind of a love story.

You're playing two Kevins in this episode: the assassin and the president. Is there a vastly different approach to the different versions of the character, or do you really view them as the same man just in two different circumstances?

I view them as the same man with two different objectives: one has the objective of getting the story, and the other has the objective of killing the president. I thought that was sort of a fun thing that Damon interjected, that Kevin's obviously a very good assassin, and the idea he's going to kill himself — as I was reading it I kept thinking, "Oh my god, is he actually going to shoot the president?" And it winds up being that the assassin is the one who gets killed. Their interior lives are the same.

In order to access the presidential bunker, Kevin needs to present his "unique biometrics." Is Damon Lindelof just screwing with you at this point?

Yeah, he's just f—king with me. Absolutely. (Laughs.) He knows it makes me deeply uncomfortable, so I can't help but think he was chuckling as he was pecking at his keys, going, "Wait until Justin gets these pages."

The foley guy deserves a raise.

(Big laugh.) I know! I noticed that too. It's like, are they putting lunch meat on this thing? Is that a ham? (Laughs.)

Kevin names Patti Levin the Secretary of Defense. Your scenes with Ann Dowd have been among the highlights of the series. How would you describe your spark with Ann, and what did you think of your final scenes together?

That's another less-than-benevolent trick Damon keeps playing on me. He knows my deep love of Ann Dowd. He obviously shares that love with me. He's now taken her away from me three times, for real this time. In the cabin in season one he had to call Ann and say, "We're killing you off the show." And I would immediately protest, "We gotta get her back on the f—king show. This is ridiculous." She's just one of the best actresses we have on our show, along with Carrie ... I mean, they're all great actresses. And he said, "Okay. If I can think of a way to bring her back." So he brought her back in the second season, and of course, we had our last scene again in the well. After "cut," we were hugging and crying and saying goodbye to each other because we weren't going to see each other next season. And then he's brought her back again. He's basically put down the banana peel three times for both of us and killed her off, and each time we thought she was really dead. The scenes with her, I just love working with her. We adore each other in real life. Needless to say, she's a fabulous actress.

The decision to bring her back last year, and the way she was brought back, really is the juice that gives season two its feel. The relationship between Kevin and Patti was so central to last season's success.

Yeah. And in a weird way and obviously different sense, that's a love story. It's two people who under completely different circumstances would have been best friends, you know? It's just that they began as adversaries and ended up becoming true best friends, as evidenced by her coming back this third time to shepherd me toward the killing of my lesser self.

And you nuke the world together! Which would be an unhappy ending for the show, were it the real world...

Even in this this world, it's now a strict possibility, with Trump telling people where the nuclear subs are, ironically.

Not hard to imagine he might believe in the three-headed monster who lives inside the volcano.

He definitely does. For sure. (Laughs.) I took that scene, the nuking of the world and why he decides to push the button, as this netherworld or whatever — whether it's the hotel, Melbourne, the president's bunker — it's a place he wants to nuke so he never, ever comes back here.

No more bags over the head.

Exactly.

When they trot out the vice president, and she's described as a "she," my mind immediately went to Laurie, given last week's episode. It's Meg (Liv Tyler), instead. What's more, we don't see Laurie in this world. Should that give us some hope she might be okay?

I don't know about that! (Laughs.) I have my own opinions. The beauty is, we won't ever find out, I don't think. I don't think we're ever going to find out. I think she killed herself. But for people who want to believe that she came up for air? They're welcome to believe that.

Your final scene with Amy was so powerful. What do you remember about closing the book on that relationship? Even if Kevin and Nora is the central love story, there's still a very real love that exists between these two exes.

I think it's that love that in ideal circumstances can only exist between exes if there's deep respect and admiration. Obviously, it's a platonic love. It's the best of circumstances, where you can actually be there for each other in a way that's freed from old history.

Clearly, you can't say what we're getting into next week with the finale...

I can't. In fact, I've misdirected you a couple of times in this conversation.

Fantastic! That's great. Can you weigh in on where we're going, emotionally?

Anything I say will taint the viewing of it. I can just tell you that I do think it's... I haven't seen it yet, but I think it's one of our strongest episodes ever. When I read it, I was enormously gratified. I thought it was a perfect last stroke on the painting for our show. A perfect punctuation.

What did you think of the penultimate episode of The Leftovers, and what are your predictions for the series finale?