Daniel Radcliffe thinks it's his "short man syndrome" which gets him through hard shoots.
The Harry Potter actor had to spend three days in a cold tank of "Saturday morning children's show" gunk to film a scene for The Woman In Black in which his character dives into the marsh to try and recover a body.
To get through the scene, he said: "You have to keep in your mind how great it's going to look.
"Basically, it's short man syndrome. I'm in here and I'm now going to prove how tough I am. I'm going to be in here for two days and dammit, I'm not going to complain once. That's the kind of pig-headed attitude that sees me through!"
He went on: "I thought it was going to be really cool, I was getting all psyched up to break the surface like Apocalypse Now, but when I came up, I was informed I looked more like Al Jolson."
Cast as Harry Potter in 2000, when he was just 10, Daniel acknowledges most of his childhood happened on set, but insists he hasn't missed out on anything.
"I still got a very good education, it was just a different way of growing up," he said.
Appropriately, he read the script for The Woman In Black just four hours after shooting wrapped on the final Potter film, Harry Potter And The Deathly Hallows: Part 2, which was released last summer.
While it took a while to "recalibrate" on a film set that wasn't Potter-related, he said it was refreshing: "Just to see my name on the call sheet next to a different character's name was a thrill."
:: The Woman In Black is in cinemas now.