Justin Theroux on His Bromance With Woody Harrelson: 'I'm in Competition' With Matthew McConaughey (Exclusive)

Theroux and Harrelson star in 'White House Plumbers,' which debuts May 1 on HBO.

Matthew McConaughey's got some stiff competition in the bromance department with Woody Harrelson, and it's a familiar face: Justin Theroux.

Theroux spoke to ET on Monday night at 92NY for the New York City premiere of his and Harrelson's highly anticipated limited true-crime series, White House Plumbers, on HBO and he talked about the strong bond he's formed with Harrelson.

"He's down to have fun all the time," Theroux says. "He's a good time. He doesn't take things too seriously. He just kind of shows up and, you know, he's very relaxed. So, it fosters a good time onset."

It's also easy to see why a bromance is almost unavoidable.

"We did have a bit of a bromance," Theroux says. "I mean, I'm in competition with McConaughey, who I guess is the OG bromance, but I'm trying to peel them away and see if I can, you know, have an open bromance."

After all, Theroux notes, "it is 2023, we should be able to share bros."

Breaking up that bromance might prove tougher if it's confirmed that McConaughey and Harrelson are, in fact, half-brothers. During an interview on Kelly Ripa's SiriusXM podcast, Let's Talk Off Camera, the Greenlights author revealed that his mother, Mary Kathlene, "knew" (emphasis, McConaughey) Harrelson's dad, Charles, back in the day, inferring that they hooked up.

Nina Westervelt/Variety via Getty Images

McConaughey, who is developing a docuseries with Harrelson exploring their relationship and dubbed Brother From Another Mother, joked that a DNA test might have to happen to get to the bottom of this family matter.

And if it turns out they are related, Theroux thinks that "would be great."

"I mean, I'm sure that they could both go spit in a cup somewhere and solve this pretty quickly," he quipped. "I'm gonna put my spit in a cup and just take my chances. Who knows?!"

That they might be half-brothers doesn't surprise Theroux, who sees similarities. He's worked with McConaughey in the past (in 2008's Tropic Thunder), so he would know.

"I can't tell whether it's just, like, their sort of slow Southern and Texas drawls or I think Texas figures in there somewhere," he says. "It's just a mentality. They're both so wonderfully sort of blissfully relaxed all the time."

For his portrayal of G. Gordon Liddy in White House Plumbers, Theroux had to grow out his mustache for six months. And by the time he wrapped, he was excited to break out the clippers.

"I wore that for six months. That was tough, tough," he says. "It was an easy thing to take off. I was excited by the end of it."

Justin Theroux

Theroux says he sported the mustache out to dinner and other social events, but while he grew it, the mustache didn't grow on him.

"It literally grew on me, and then it did not grow on me as just something that I would want to have in my daily life," he explains.

Theroux also shared that, during early testing, there was a big debate on whether he should grow out the mustache or have a fake one that he could simply wear during filming.

"Nothing beats just having the damn thing on your face, you know?" he says. "So, I grew it out."

But the minute they wrapped, the 'stache was a goner.

The five-part limited true-crime historical drama will look back on the Watergate scandal and the downfall of Richard Nixon's presidency. Harrelson stars as E. Howard Hunt. According to HBO, White House Plumbers, which is directed by David Mandel and written by Alex Gregory and Peter Huyck, all former members of the Veep creative team, "sheds light on the lesser-known series of events that led to one of America's greatest political crimes."

The satirical series "begins in 1971 when the White House hires Hunt and Liddy, former CIA and FBI, respectively, to investigate the Pentagon Papers leak. After failing upward, the unlikely pair lands on the Committee to Re-Elect the President, plotting several unbelievable covert ops -- including bugging the Democratic National Committee offices at the Watergate complex."

White House Plumbers debuts May 1 on HBO.