The Puerto Rican actor plays a nobleman who transforms into a vicious wolf in the upcoming thriller - but he admits he was far from menacing after heavy prosthetics were applied to his face and body every day.
And after spending three hours in the make-up chair, Del Toro required an entire team of assistants to help him speak aloud.
He explains, "I needed a team to help me if I wanted to say something because I had to take off my teeth. They would take off my hands so I could scratch myself or eat."
The gruelling filming schedule often left the star too exhausted to remove his make-up at the end of the day - meaning staff at his London hotel were given a fright upon his return in the early hours.
He adds, "I'd be like, 'Ok, I'll clean this off tomorrow - I'm exhausted.' So I would walk in and look like I'd just been beaten to pieces at 5am. The hotel guys already know but the first time I'm sure I freaked them out."