Mark Ruffalo. Picture: Bang Showbiz

Mark Ruffalo will portray identical twins in a new six-part HBO series, 'I Know This Much Is True'.

Mark Ruffalo will play twins in a new HBO series.

The 50-year-old actor will also serve as executive producer in the six-part programme 'I Know This Much Is True', which is based on the book of the same name by Wally Lamb, and he can't wait to get started.

Mark - who won awards for his performance in Ryan Murphy's HBO movie 'The Normal Heart' - tweeted: "Excited to be working with the @HBO team again! Can't wait for you all to see this story."

'I Know This Much Is True' is described as a family saga, which follows the lives of identical twins Thomas and Dominick Birdsey in a story of "betrayal, sacrifice and forgiveness" in 20th century America

The author will also executive produce, along with Mark and the series' director, Derek Cianfrance.

Mark has been kept busy of late working on the fourth 'Avengers' movie - in which he returns as Hulk - and recently admitted reshoots mean he and his co-stars don't know how the film will end.

He said: "We don't even know what it's gonna be yet. We're not just doing reshoots. We're going to finish the movie, which we really didn't get to finish totally when we left it last year."

In fact, Mark suggested that the directing duo - Anthony and Joe Russo - are currently uncertain as to how the movie will end.

He added: "I don't even know that [the Russos] really know exactly. 

"Some of it is happening while we're there. It's pretty amazing. And we'll shoot some stuff and a few days later come back and reshoot it 'cause we wanna take it in another direction. 

"It's a very living organism; even as we approach it being a locked picture, we're still working on it."