"It was great to have something that was meaty enough to get my teeth into, but let other people do the heavy lifting." Firth told journalists at the Venice Film Festival. "It's basically all I did last year and it suited me very well."
Firth - who won the best actor Oscar for The King's Speech and was nominated for the same award for A Single Man - plays the calm, collected intelligence agent Bill Haydon, a counterpoint character to the film's lead Gary Oldman, playing the main character, retired spy George Smiley.
Tinker, Tailor, Soldier, Spy, directed by Tomas Alfredson - the Swedish director of 2008 vampire film Let The Right One In - is among entries vying for the top Golden Lion prize at the festival's conclusion this Saturday.
Alfredson's interpretation of the John Le Carre Cold War spy yarn is a slow-brewing, elegant retelling of the classic novel.
Firth was asked whether audiences would fall for a subtly paced spy drama that unravels to reveal betrayal and retribution without the sexy antics of a James Bond film, or the action packed scenes of a Bourne Identity.
"There is a tendency to underestimate audiences," Firth said at a news conference. "People don't just want 'slash and burn' - so I am optimistic about (the film) having an enormous audience."
Audiences may be surprised that Firth opted for a supporting role - and less screen time - given his domination of the screen as King George VI in The King's Speech. Likewise, in A Single Man in 2009, Firth's character, a homosexual contemplating suicide after the death of his lover, monopolises the story - a performance that won him the best actor honour that year in Venice.
Oldman's mannered performance as Smiley offers an anchor in this complex whodunit that focuses on the inner workings of the elite MI6 intelligence agency, which needs to cover up - or prove - the existence of a highly placed Soviet mole.