The actor - who plays CIA counterterrorism director David Estes in Showtime drama Homeland - explained that even a lot of English fans don't realise that he isn't an American.
"You've been acting for 27 years, and people say 'I didn't know you were from Birmingham!' It's quite surprising," he told Lorraine Kelly today (November 27).
Talking about his American accent for the role, he said: It's a very difficult accent for me to maintain. The voice I do is very specific for that character."
On his character's storyline in the show, he said: "He's an interesting character. Is he a baddie? People think he's a baddie. Nobody knows. You may find out."
When asked about the show's success in the US and the UK, he added: "I didn't realise it would be quite so big here. We get bigger audiences here than in the US.
"My mum rings me after every episode, saying, 'What's happening? Who's he?' It's funny, because I can't say anything."
Earlier this year, David Harewood said that there are better TV roles for black actors in the US than the UK.
Homeland was renewed for a third season of 12 episodes by Showtime last month.