The two stars play friends-turned-rivals Sigmund Freud and Karl Jung in the new David Cronenberg film and Mortensen admits his castmate had an odd way of warming up.
Appearing on The Late Show With David Letterman on Friday (13Jan12), the Lord of the Rings star explains, "I've seen a lot of things but I've never seen this. Before every shot he had a thing that he'd do because he wanted to be very, like, on the verge of leaping at you... and so he'd jump around on one foot.
"Sometimes, he'd be, like, skipping rope and then he wanted to have this very lean, angry look, intense... so he'd cover an eye... and he'd do this for a while.
"I didn't know what to make of it. You can't see it in the movie but every time I did a scene with him he'd take his hand away (at the) last minute and he's be cross-eyed... (and) mouth breathing."
Mortensen admits there was one scene that particularly unnerved him - when Fassbender started his one-eyed hopping routine in a small boat.
He adds, "He starts doing this and I'm thinking, 'He's gonna put a hole right in the bottom of the boat.' It was really scary. He's a fine actor though. It worked for him."