The actor is Doug in The Hangover Part II, which is the sequel to the hugely successful 2009 comedy movie.
The original saw a group of men embark on a wild bachelor party in Las Vegas, and this time round they up the ante by travelling to Thailand.
Bradley Cooper, Ed Helms and Zach Galifianakis also reprise their roles, and Justin has admitted he and Ed went to great lengths to research their scenes.
They expected part of the plot to see the men spending time in sex clubs, so decided they had better check some out ahead of shooting.
"I saw some dirty (stuff) in Thailand that got burned into my eyes. I honestly can't say what because it is so disgusting I feel ashamed for going," Justin told Bullett Magazine. "I shouldn't have gone."
Justin refused to open up about the experience at all for fear of offending people.
The 32-year-old admits it's one of his biggest regrets, and issued a stark warning for anyone considering following in his footsteps.
"It is so far beyond what people think," he said.
- from stuff.co.nz