The actor played the chimpanzee in the 2011 sci-fi movie and its sequel, 2013's 'Dawn of the Planet of the Apes'. He claims he was relieved that the ape was older in the second movie because he couldn't physically handle putting as much "extraordinary pressure" on his "shredded" knees for a second installment.
He said: "I was so thankful that playing Caesar this time around he was more bipedal, I'm sure that was informed by the fact that my knees are shredded.
"I think the most physically challenging role - apart from Gollum - was actually Caesar as an infant because the pressure on the knees was pretty extraordinary. Doing that hour after hour I was screaming in agony."
Playing small characters like Gollum and Caesar has left him unable to move follwing lon scenes.
Speaking at a Q&A to launch the 'Dawn of the Planet of the Apes' Blu-ray and DVD - which is available from Monday (24.11.14) - he added: "When I was training for Caesar I actually bumped into an actor friend of mine, I was jogging furiously, it was Danny Mays.
"He'd just landed this job in South Africa and he was about to move all this family out there. I bent down to talk to him at his windscreen and I was telling him to go for it. But when he drove off I was literally stuck on the pavement. I couldn't move!"
'Dawn of the Planet of the Apes' is out on Blu-ray, DVD and to download on November 24.
Tagged in Andy Serkis