Dennis Quaid gives ET an update on his twins and talks about his 35-pound weight gain for his new HBO film, "The Special Relationship."

The movie focuses on the bond between former President Bill Clinton and Tony Blair, played once again by the talented Michael Sheen -- who also took on the role of the British Prime Minister in the film 'The Queen' with Oscar-winner Helen Mirren and in the TV movie "The Deal."

"It was the most daunting role I've ever taken on," Dennis says of playing the former president. He gained 35-pounds for the role and says he relied on Clinton's biography to help him get into character. "It was like having him in the room talking to me," he says of the tome. The two know each other and the actor even spent time at the White House when he was in office.

On a personal front, Dennis is continuing his work to help reform health care. "Since the overdose of our twins, we formed a foundation," he says. While the twins are doing well, "there are 100,000 people who die of accidents that happen in hospitals."

Watch the video for more -- then see Dennis in his most challenging role yet when "The Special Relationship" debuts on HBO on May 29.