Stallone left the Finnish filmmaker feeling a little nervous when he confessed to his phobia on his first day in the Italian Alps after checking out the set's 11,000-foot (3,352 metre) base camp.
Harlin tells Entertainment Tonight, "I said, 'That's where our set is,' and he's like, 'So who's going to go up there?' I said, 'You are.' He looks at me and he says, 'You gotta be kidding. I have a fear of heights. The highest I will ever go is the heels of my cowboy boots!'
"It became my task to trick him into wanting to be higher in the mountains... I knew his psychology and I put him in a situation where it would have been embarrassing for him to, in front of the crew, refuse to get there. Obviously, I was able to get him there, and then once I got him to relax he was more than willing to do everything, and even more than I wanted him to do.
"The highest peak that we worked was 13,000 feet high, and there he was right on the edge, basically doing whatever it took."