United Kingdom, April 6 -- John Lithgow admits playing Albus Dumbledore until he's almost 90 is "an extremely difficult thing to contemplate".

The 80-year-old actor has signed on to play the Hogwarts headmaster in HBO's upcoming Harry Potter reboot, which will start this Christmas and run for seven seasons, with each one covering one of JK Rowling's books in the wizarding saga.

Speaking in new documentary Finding Harry: The Craft Behind The Magic, he said: "I knew that when I did the first season of Harry Potter, I would be turning 80 years old.

"That meant that I would age about 88 before it was all over. This is an extremely difficult thing to contemplate."

He also reflected on the journey awaiting leading trio Dominic McLaughlin (H...