Mark Ruffalo did roughly 500 pushups a day while filming his new HBO series I Know This Much Is True

HBO aired the sixth and final episode of its adaptation of the Wally Lamb novel I Know This Much Is True on Sunday night, with writer-director Derek Cianfrance revealing some interesting details about star Mark Ruffalo.

Ruffalo plays both Dominick Birdsey and his identical twin brother Thomas Birdsey in the series, with Cianfrance telling The Wrap how they split the filming up between both characters.

When portraying Dominick, Cianfrance said Ruffalo was doing 500 pushups a day to help Mark get ‘in the zone’ to play this character.

Details: HBO aired the sixth and final episode of its adaptation of the Wally Lamb novel I Know This Much Is True on Sunday night, with writer-director Derek Cianfrance revealing some interesting details about star Mark Ruffalo

Cianfrance explained that they spent the first 16 weeks of production filming his scenes as Thomas, where Ruffalo had to drop 20 pounds by going on a strict 1,000-calorie-a-day diet.

‘I guess you could say he was always a little bit hangry. He was eating an egg white for breakfast and just starving,’ Cianfrance said.

After that segment of the production, he would then put on 30 pounds to play Thomas, who struggles with schizophrenia.

Diet: Cianfrance explained that they spent the first 16 weeks of production filming his scenes as Thomas, where Ruffalo had to drop 20 pounds by going on a strict 1,000-calorie-a-day diet

Diet: Cianfrance explained that they spent the first 16 weeks of production filming his scenes as Thomas, where Ruffalo had to drop 20 pounds by going on a strict 1,000-calorie-a-day diet

The writer-director added he had an ‘instinct’ one day to have Ruffallo drop and give him 50 pushups and then deliver his lines.

‘He was a little out of breath, you know? He was smoking these cigarettes and struggling for his breath a little bit, and his chest was puffed out, and I noticed the vein in his neck was engorged,’ Cianfrance said.

When the director asked him to do another 50 pushups before delivering another set of lines, Ruffalo questioned his reasoning, but Cianfrance explained.

Instinct: The writer-director added he had an 'instinct' one day to have Ruffallo drop and give him 50 pushups and then deliver his lines.

Instinct: The writer-director added he had an ‘instinct’ one day to have Ruffallo drop and give him 50 pushups and then deliver his lines.

‘I was like, “Well, you’re just becoming Dominick — this guy who’s just struggling for every breath. This guy with a barrel chest,”‘ Cianfrance said. 

‘So we shot for 17 weeks and I would have Mark do push-ups — he was doing probably 500 pushups a day,’ the writer-director added.

He said Ruffalo did pushups ‘all the time between takes,’ adding, ‘it was always our way to get Mark into the zone as Dominick.’

Becoming Dominick: 'I was like, "Well, you¿re just becoming Dominick ¿ this guy who¿s just struggling for every breath. This guy with a barrel chest,"' Cianfrance said

Becoming Dominick: ‘I was like, “Well, you’re just becoming Dominick — this guy who’s just struggling for every breath. This guy with a barrel chest,”‘ Cianfrance said

The series follows Dominick Birdsey while he recounts his troubled relationship with Thomas, his paranoid schizophrenic twin brother, and his efforts to get him released from an asylum.

The cast also includes Rob Huebel, Kathryn Hahn, Melissa Leo, Rosie O’Donnell, Archie Panjabi, Imogen Poots and Gabe Fazio.

Ruffalo will return as The Hulk in the new Marvel Studios series What If?… and he has signed on to play 60 Minutes producer Don Hewitt in Newsflash.

Series: The series follows Dominick Birdsey while he recounts his troubled relationship with Thomas, his paranoid schizophrenic twin brother, and his efforts to get him released from an asylum

Series: The series follows Dominick Birdsey while he recounts his troubled relationship with Thomas, his paranoid schizophrenic twin brother, and his efforts to get him released from an asylum

Cast: The cast also includes Rob Huebel, Kathryn Hahn, Melissa Leo, Rosie O'Donnell, Archie Panjabi, Imogen Poots and Gabe Fazio

Cast: The cast also includes Rob Huebel, Kathryn Hahn, Melissa Leo, Rosie O’Donnell, Archie Panjabi, Imogen Poots and Gabe Fazio

Coming soon: Ruffalo will return as The Hulk in the new Marvel Studios series What If?... and he has signed on to play 60 Minutes producer Don Hewitt in Newsflash

Coming soon: Ruffalo will return as The Hulk in the new Marvel Studios series What If?… and he has signed on to play 60 Minutes producer Don Hewitt in Newsflash