The Totally Not Creepy Reason Zach Galifianakis Stole Justin Bieber's Underwear
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The actor spills on swiping Bieber's drawers and the scary moment he thought Sean Penn would "deck" him filming "Between Two Ferns."

As if Zach Galifianakis' antics on his "Between Two Ferns" series aren't wild enough, the behind the scenes stories he just shared with David Letterman are even more ridiculous.

The "Hangover" star opened up about his purposefully awkward celebrity interview videos during a bonus episode of Letterman's "My Next Guest Needs No Introduction," which was filmed at a Netflix event last month and is currently available to watch on the streaming platform.

Galifianakis said the idea behind the show was "to be a little ruder or a little bit experimental in the way I talked to people" after previously hosting a more generic (and short-lived) talk show for VH1. Though he pleaded with Dave and Netflix to not include his story about Justin Bieber on the edited version of the episode, they did -- and we're extremely grateful.

"Can I tell you something I would ask Netflix not to put in this thing?" he began. "After I did this video, I got slimed. I had to change clothes because I had a show that night and I had to do standup and I had green slop on me. I can't believe I'm telling this story. This is special for the audience only."

"I go to his trailer to get this stuff off of me, he's gone," Zach continued. "I didn't have any underwear, I didn't bring any underwear, I brought pants and a shirt and I can't wear this underwear. So I see a pair of underwear in the corner. I needed a pair of underwear. But the fact that it was his was probably more interesting to me. I put 'em on and then I started thinking about it, on a lot of levels it's wrong."

When Letterman noted it was an "emergency," Galifianakis agreed. "It has nothing to do with wanting to put Justin Bieber's underwear on," he added, "I'm questioning myself, I'm not 100% sure why I did it. I took them off quickly."

While he didn't continue to wear them, Zach did swipe them -- "because I have nieces," he explained.

"I kept it so I could ..." he began, before Letterman broke out laughing. "Oh no, 'Girls, guess what, Justin Bieber's underpants,'" Dave added.

"I thought she'd be excited," Zach said of his niece. "I was like, 'Hey, I've been saving these for you' and the look on my niece's face. She did not accept it. She was not into it at all. Which, I'm glad that she wasn't. My sister was excited like I was. She was into it."

Zach also explained that while his guests on the mock interview show know what they're signing up for, they aren't made aware of the questions ahead of time. One time, that really backfired for him.

"I still have to ask questions they're unaware of and that can be tricky and it's been tricky a couple times," he said, before sharing an example. "Sean Penn, I stopped that one real early. I'll never forget, I said something to him and he's either a really good actor or he was being completely honest and I couldn't tell, so I walked up to the director like, 'We're done.' He goes, 'We don't have anything.' I said, 'We're done, we have a lot.'"

"I think maybe I was threatening to beat him up in a very believable way, as the twin brother version of myself," he explained. "At that moment, I thought he was gonna deck me, I really did. If I remember correctly, I went to dinner with Sean that night, I think he was just acting. I didn't want to know."

You can see the heated moment below: