Freeman plays Bilbo Baggins in Peter Jackson's forthcoming Hobbit film, and Cumberbatch supplies the voice of the dragon, Smaug.
But Cumberbatch told the Radio Times: "As an actor, you can do weight loss, weight gain, put on silly noses, crazy accents, move like a dragon, inviting people to look at the fireworks and admire how different you're being.
"But with acting like that, it's all about look-at-me, when what you should be doing is helping the audience care about the person they're watching."
Freeman, who also stars in Sherlock with Cumberbatch, said: "I always think it's a bit of a red herring, this business of whether you're pretending to be something different.
"I mean... show me someone who's being nothing like they are in real life and I'll show you a rubbish performance.
"In every film (Robert) De Niro's in, he's the best actor on God's earth, but he'll always pull the same De Niro face at some point. You're never going to go, 'Oh, I didn't know who that was'."