Shooting the new HBO miniseries I Know This Much Is True was unlike anything Mark Ruffalo had ever done in his career. Ruffalo plays twin brothers, which meant playing two characters, but that was just the start of the challenge. Though they’re twins, there is a 30-pound difference between the brothers, so Ruffalo first shot all of his scenes as Dominick Birdsey, then took five weeks to gain the weight he needed to play Thomas. It wasn’t easy to do, and Ruffalo told us it required a lot of planning to get all of the logistics right. (Click on the media bar below to hear Mark Ruffalo)

I Know This Much Is True airs Sunday nights on HBO.

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