L-R: Ewan McGregor in Fargo and Trainspotting 2.

Ewan McGregor played a guy and his clone in The Island, and Jesus and Satan in Last Days in the Desert. But taking on the dual roles of the Stussy brothers in the upcoming third season of Fargo, premiering April 19 on FX, is presenting unprecedented challenges.

For one, Ray Stussy, the younger brother, is heavier than his older sibling, Emmit, which could have been easily addressed with a fat suit and prosthetics. But creator and showrunner Noah Hawley had other designs. “You need to put on weight,” McGregor recalls Hawley saying to him at a restaurant in Los Angeles last October, three months before production began. At the time, McGregor was 45, and at his fittest. He had just finished filming Trainspotting 2 in Edinburgh, Scotland, where he and cast member Jonny Lee Miller regularly exercised, running around and up an imposing mountain called Arthur’s Seat.

But McGregor didn’t protest. “I ordered a massive dessert and started putting on weight from that second onward,” he said during a press conference call Thursday. “From October until January, when we started filming, I just started eating whatever I wanted. I made sure that I had carbs with everything and French fries with everything. I didn’t have any technique other than eating a lot. I think if you spoke to a dietician, I probably did it all wrong.”

He never weighed himself, but he did have to buy new Levi’s that were three inches larger in the waist. “It’s quite nice when you’re ordering — you can order whatever you like,” he says. “But the truth is I would go to bed every night not feeling very great. I’m a small guy. I’m not really used to carrying weight. It doesn’t make you feel great. I like to feel fit and healthy. But it was effective. It worked.”

When McGregor read the first script, he understood why it was important that he put on some pounds. Ray and his girlfriend Nikki (Mary Elizabeth Winstead) have a bathtub scene, and Ray has to get out of the bath. “You see my naked body. Although I wear some padding for Ray to make him heavier, this scene had to justify that padding,” he explains. “It wouldn’t make sense for me to wear padding and not be fat when I got out of the bath. So when I got out of the bath, I was properly overweight for me.” (It takes him two-and-a-half hours to transform into Ray — padding, prosthetics, wig, cowboy boots, and all.)

But what about Emmit, who is meant to be a leaner man? To play the rich elder brother, McGregor had to sport Spanx. “It’s a really unbelievably effective compressing T-shirt that I have help getting into and help getting out of, and it makes Emmit less heavy-looking,” he says. “The Spanx makes me hold my body in a different way because it’s tight and keeps me more upright, I suppose.”

As production has progressed — it ends May 5 — McGregor has lost some of the weight. “Because of the story line, and the nature of wearing padding for Ray, I’ve been able to lose that weight and it’s been quite helpful for the story with Emmit,” he adds. “That’s come about by chance, but the fact that he gets more gaunt-looking in his face has been quite useful for the story. It’s worked out quite nicely.”

Perhaps the trickiest part of playing both roles roles, McGregor says, has been nailing the specific Fargo accent and giving it enough nuance for both characters. Many times during production, he’s had to play both men in a single day. “It’s probably difficult for an American, too, but for a British person, there are sounds that sound Scottish and that can lead me to think I’m getting it wrong. Some of it sounds Irish. I sometimes feel I’m doing a bad Irish accent. Even though they’re similar because they’re brothers, hopefully they sound like two different people.”

McGregor says he enjoys playing both the “soulless” Emmit, who made a fortune as “the parking lot king of Minnesota,” as much as more “lovable” Ray, a parole officer. But the crew has a clear favorite: “The crew prefers Ray,” he says. “They really like it when Ray is on set, probably more so than when Emmit is.”

Ewan McGregor on Gaining Weight for Fargo