While the naïve and lonely Thurman Merman is the heart of the Bad Santa franchise, actor Brett Kelly concentrated on his character’s stomach while prepping for the second film in the Billy Bob Thornton-starring series.

“I put on 40 pounds before the start of the shoot,” he says. “I ate anything I could get my hands on. But then it was two months in Montreal — the greatest town for disregarding your health — so I put on another 15 during the shoot. It was 50, 55 pounds by the end. The fun part was before, putting on all the weight, and doing the movie. The pain came after. It took about four months to put on and then I’d say about twice that to take it all off.”

The Vancouver-born Kelly was around six when he scored his first professional gig — a Huggies commercial — and only eight when he starred opposite Billy Bob Thornton in 2003’s foul language-filled Bad Santa. “Everyone thinks I was scarred for life from Billy Bob,” he says. “But I had five or six auditions for the first one, and I’m running lines with my Dad. Once your dad is calling you all this stuff, it was a f–king piece of cake to have some stranger do it. I know a lot of it went way over my head, where I just didn’t even realize what was going on a lot of the time. Even a couple of years later, when I was 11, 12, watching the movie, I still didn’t know what a lot of the jokes were. That was a lot of fun watching it, when I got older, and seeing a lot of the stuff that I’d missed at that time. But as a kid you just do what you’re told, go where you’re told.”

After Bad Santa, Kelly was cast in another holiday-themed cult classic, the 2007 horror anthology Trick ‘r Treat. “I remember [for] the vomiting scene, eating 30 Mars bars in one night,” he says. “I never thought I’d hate chocolate, but that did it.” He was also cast in a couple of less-well-remembered sequels: 2005’s The Sandlot 2 and 2008’s Slap Shot 3: The Junior League. “Halloween, Christmas, straight-to-DVD — that’s right in my wheelhouse!” he laughs.

At the age of 17, Kelly took a break from the film business and spent four years studying business at the University of British Columbia. “This line of work, you never know what’s going to happen,” he says. “I always wanted to have something to fall back on.”

But Kelly hasn’t used his bachelor’s degree yet. “I had my last ever exam and two days later I got the call about Bad Santa 2,” he says. “So, it was meant to be, I guess.” Thornton recalls that reconnecting with his Bad Santa costar was an initially strange experience. “The first time we met about the script, he flew down to L.A., and here he comes, he looks exactly the same almost, except he’s just bigger,” the actor says. “I told him, ‘Dude, this is weird seeing you like this.’ He’s graduated from business college, and he ordered a beer, and I’m like, ‘I don’t know if I can handle this!'”

In Bad Santa 2, we find Merman working at a sandwich shop. Kelly recently spent a day delivering bread-based edibles to New Yorkers who tweeted at the film’s account. “One guy came stumbling out in his underwear,” he recalls. That sounds very Bad Santa-ish. “Oh, yeah. It fit very well.”

Kelly says he plans to move to Los Angeles permanently this January to pursue acting as a career. “I’m giving it a good go,” he explains. “I don’t want to have to use the degree!”