Zach shares two sons, aged four and seven, with his wife Quinn Lundberg and the children have no clue their father is a famous actor and comedian.
"My kids think I'm a librarian somewhere. They don't even know. They think I'm an assistant librarian somewhere," he tells Entertainment Tonight.
He also jokes his boys "shall never know" about his role as a prankster who spikes his friends' drinks with LSD in the hit 2009 comedy "The Hangover".
"I'll just deny it. I'll say, 'I don't know what you're talking about. I had nothing to do with that movie,' " he laughs.
Although he admit's he'll probably show the film to his sons one day, he's in no rush, explaining they "need to mature a little bit before they see that."
The charade might be difficult to keep up, though, as fans often tell Zach that they - and their children - love the movie.
"People used to come up to me and be like, 'Oh, my kids love 'The Hangover',' and I'm like, 'You're a terrible parent,' " he adds, admitting he's happy to protect his boys from the "tricky" part of his life - fame - while he can.
"You wanna protect their innocence as much as possible," he notes. "They're just children."