Mark Ruffalo did a series of push-ups before most takes on the set of HBO miniseries I Know This Much Is True to help him get into the right mindset.

The Avengers star lost 20 pounds (nine kilograms) to play the lean and wiry Dominick Birdsey for 17 weeks of the production, before putting on 30 pounds (13 kilograms) to play Dominick's twin brother Thomas, who suffers from schizophrenia.

Director Derek Cianfrance suggested the actor to did push-ups before a scene to get into the mindset of the stressed and breathless Dominick, and the technique worked so well Ruffalo ended up doing around 500 per day.

"Mark is the kind of actor where he'll do anything. So, he dropped to the ground, he did about 35 push-ups, he came up, and he did this phone call scene and... he was a little out of breath, you know?" Cianfrance recalled to The Wrap. "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. We finished the scene and I was like, 'Hey, let's do another take. Can you do another 50 push-ups?'

"We shot for 17 weeks and I would have Mark do push-ups - he was doing probably 500 push-ups a day. All the time between takes, it was always our way to get Mark into the zone as Dominick."

To make matters worse, the 52-year-old was also embarking on a 1,000 calorie-a-day diet, so he was always "hangry" - hungry and angry - on set.

"I guess you could say he was always a little bit hangry," Cianfrance quipped. "He was eating an egg white for breakfast and just starving. And he couldn't eat for another three hours, and all he could eat then was a granola bar that had 120 calories or something, because he was on this strict diet. It created this guy Dominick who was just wiry and aggressive."