Mark Ruffalo is joining the elite club of actors who have portrayed twins or clones, thanks to his latest role(s) in HBO's adaptation of Wally Lamb's popular novel I Know This Much Is True. Ruffalo will play identical twin brothers Dominick and Thomas Birdsey, and the methodology behind Ruffalo's transformative turns in the limited series is truly fascinating.

Ruffalo first portrayed Dominick, the more mentally stable and socially adjusted twin. Then, he shaved his beard and gained 30 pounds and came back more than a month later to shoot as Thomas, the twin who has schizophrenia.

"We didn't want it to be like I walk on and then do the same scene the same day," Ruffalo told journalists Wednesday at the Television Critics Association winter press tour. "We took six weeks off to really separate these two guys. Thomas has a lot of medication, and these mood stabilizers and antipsychotics cause people to put on weight. It was really important for us to really, really turn these guys into different people."

Ruffalo is executive-producing I Know This Much Is True alongside Derek Cianfrance, who also wrote and directed the six-part limited series. Cianfrance compared his technical approach to that of filmmaker Michael Mann.

"We have an amazing actor in Gabe Fazio, who is playing both parts as well," said Cianfrance. Fazio plays opposite Ruffalo in scenes with both characters. "He doesn't necessarily look like Mark but Gabe shot the scenes and played that role like we would shoot any scene. What I didn't want to do is have the technical challenges overwhelm the story and the characters we were trying to create."

Cianfrance added, "I was taking inspiration from Michael Mann's Heat where there's that great scene where Robert De Niro and Al Pacino have dinner at night, and in that scene, they're never seen on screen together," Cianfrance added. "And I thought to myself that was a stroke of confidence from a filmmaker. He was creating the flip sides of a coin. You can't see a head and the tail at the same time. It's impossible, and so that became a real kind of aesthetic."

When asked if he'd seen his Marvel pal Paul Rudd's recent turn playing clones in the Netflix film Living With Yourself, Ruffalo said he had, and that they chatted about playing dual roles.

"I didn't talk to him before I did this, but we talked a little about it afterwards," Ruffalo said of Rudd. "We didn't really compare notes. But he's great in that. He's great in everything, goddamn it."

Like the book on which it's based, I Know This Much Is True is a familial drama that deconstructs Dominick Birdsey's life and relationships with his mentally ill twin brother, unhinged girlfriend, and disenchanted ex-wife, all while processing the grief over losing loved ones.

I Know This Much Is True premieres on HBO this spring.