The slender 35-year-old had little time to prepare in between shooting the second season of Pablo Escobar drug drama Narcos and hitting the set of the X-Men spin-off to portray murderous cyborg Donald Pierce, so he ended up "gorging" himself to quickly bump up his weight.
"I had about five days (between roles)," he told U.S. breakfast show Good Morning America. "I was trying to slowly gain weight without anyone noticing too much (on the set of Narcos)."
Boyd added, "I gained about 20 pounds (in total); I put on about 10 pounds in a week!"
The new film serves as Hugh Jackman's final outing as the titular character and his clawed mutant alter ego, and while Boyd admits there was some pressure on the cast and crew to make the blockbuster the best of the franchise, it only made the shoot more of an adventure.
"(The pressure made it) more exciting because it was a way to go out with a bang," he said, adding of the film's plot, "It plays like a western, it really does; it's a slow burn."
Boyd also had fun working alongside child actress Dafne Keen, who tackles the role of mutant Laura Kinney in the movie.
"If anything was intimidating, that is - a 12-year-old accomplishing that much in a role!" he smiled. "It's an honour to be a part of this group of people."
Boyd wasn't the only one impressed by Dafne's movie debut - director James Mangold recently heaped praise on her talents and expressed his desire to build a full feature around her character.
"I'd love to figure out a story for that character," he told Den of Geek U.K. "She (Keen) is a remarkable child who I'm very attached to, and who I feel incredibly indebted to."
Further praising her intuition on set, James continued: "She'd be like, 'What do you want to feel?' Or she'd say, 'What am I supposed to feel?' She's like a method actor of 11 years of age (at the time of the shoot), going, 'Don't tell me what to look at, just tell me what the scene's about!'"