The actor - who has appeared as nice guys Tim Canterbury in The Office and John Watson in Sherlock among others - explained that he isn't the same sort of person away from the camera.
"I totally get it and I'm sure I would feel the same way about this actor who had played a few of those parts. Which is only positive," he told BANG Showbiz.
"It can be frustrating, just because you think, well, I know I'm not that person and you don't know I'm not that person.''
Freeman also said that he was amazed by the ratings success of Sherlock, which at one point had beaten EastEnders in viewing figures.
''Some of the viewing figures we got with the second series of Sherlock were f**king outrageous," he said.
''One week, we beat EastEnders, and I'm so proud - not because we beat EastEnders - but I'm just proud that millions, I mean literally millions of people wanted to watch it then. That night, do you know what I mean?''
Martin Freeman will soon appear as the lead character of Bilbo Baggins in Peter Jackson's first film in his Hobbit trilogy.
The Hobbit: An Unexpected Journey will be released on December 14, with subsequent instalments The Hobbit: The Desolation of Smaug and The Hobbit: There and Back Again due out in December 2013 and 2014 respectively.
Watch the trailer below: