Howard wanted the comedian to play a butler in the adaptation of Dan Brown's best-selling novel - but Gervais is sick of watching cameos wreck good films.
He says, "I was offered the part of the butler.
"I told director Ron Howard, 'I will ruin your film. The number of times I've sat down for a great film by a great director and a British actor pops up and ruins it for me.
"I don't want to be that bloke."