Stiller, Vince Vaughn and Jonah Hill star in The Watch as neighborhood guards who stumble on an alien plot to take over earth.
In an interview with USA Today, Stiller said he felt no pressure to make an authentic sci-fi movie.
"The constraints come off [in a comedy like this]. That's why we're there," he said, adding: "We're not making an action film. We're not going to out-sci-fi Ridley Scott. We just are trying to give people an escape, and you need the freedom to try any joke."
Stiller went on to say that he takes a very serious approach to his acting roles, even in his broadest comedy films.
"Even when you're doing something completely out there like Dodgeball, where you know people aren't going to consider it a masterpiece, you still have to take it seriously as an actor," Stiller explained.
He added: "Good acting is about the choices you make, even in silly escapist movies. I think the best comedy comes from intelligent people, bringing their true personae to the screen."
The Watch is now playing in the US and opens in the UK on August 24.
Watch a trailer for The Watch below: