As a director, Derek Cianfrance is usually given a major say in determining who will be cast in a project. But in the case of HBO’s adaptation of Wally Lamb’s “I Know This Much Is True,” Mark Ruffalo was already in place to star as Dominick Birdsey and his twin brother, Thomas, when Cianfrance signed on. In fact, Ruffalo, who had the option on the 1998 novel and was also set to serve as an executive producer, was the reason Cianfrance signed on.

“Mark was always one of my favorite actors. I had tried to get him in ‘Blue Valentine’ back in the mid-2000s and he instead chose to do ‘13 Going on 30,’” Cianfrance says. “We sat down to breakfast together and my answer was ‘yes’ before I even read the material.”

The bond the two formed was integral to the level of trust needed on-set because there were many scenes that featured Ruffalo as both brothers — and those sequences were shot six weeks apart. They started with Ruffalo portraying Dominick, in part because “we thought Mark could gain weight easier in those six weeks than he could lose weight,” says Cianfrance. Additionally, Ruffalo notes, “we wanted Thomas to have an institutional haircut and there was no fast way for me to grow my hair back and neither of us wanted me to wear a wig.”

Although the characters are identical twins, their different experiences over four decades shaped their physicality in the present-day part of the story. Thomas is a schizophrenic who has been in and out of institutions, and “all of that medication in him makes him bloated and changes his personality and posture,” Ruffalo says. Meanwhile, his brother is his caregiver — and one who often spirals from the stress that comes from such a position.

“Dominick, in a strange way, is harder to play,” Ruffalo says. “Thomas can be whatever he’s feeling at any given time; Dominick’s got to keep it together.”

Also integral to this process was Gabe Fazio, who stood in as Thomas when Ruffalo was playing Dominick and vice versa. Fazio, too, went through the process of gaining and losing weight to match the look and body language of the appropriate character, but his performance was ultimately erased when Ruffalo was composited into the final cut. (Fazio can be onscreen in another role, however.)

“Early on in our rehearsal, Mark used to tell me, ‘That’s not the way I would do it,’” Cianfrance recalls. “And I would tell Mark, ‘Think about Dominick and Thomas. Thomas is not acting in the way that Dominick wants him to, so use this — try to get him to behave the way you want him to behave. That’s only going to add to the chemistry of the characters.’”

Inspired by the scene in “Heat” in which Al Pacino and Robert De Niro sit across from each other in a diner, Cianfrance leaned into a shot-countershot style for scenes that featured the brothers, only showing both of their faces onscreen in the same frame in select, special moments.

He also made sure to stay as physically close to his actors as possible, including scrunching down in the backseat of a picture vehicle during a tense scene in which Thomas freaks out and sees his brother, who is driving, as a demon.

“Derek and I worked together on the scripts, on the characterization and the physicalization [and] the looks — and we were sympatico,” says Ruffalo. “It came to a point where he didn’t even need to give me a note, I could just feel what he needed from me.”