The Oscar-nominated actor explained to The Guardian that giving a credible portrayal of the stuttering monarch was a deceptively difficult task, due to the balance that needed to be achieved in order to maintain an appropriate tone for the film.
He said: "There are so many pitfalls - if you stammered the wrong way, it might be comedic and that would be a problem. If it was too painful, audiences would start to wish they were not in the cinema. If it was too severe, it would slow the film down. The stammering had never to misfire.
"The other risk was that we would run shy of it and, because of our fear about pacing, not commit to it enough. So the rhythm of the stammer was key."
Firth also admitted that he struggled to envision life as a member of the royal family, especially one with as much inner torment as George VI.
"You can't know what it is like to be a member of the royal family," he stated. "It is not the same as being an actor. You are born into it and that is everything you have known. So that was a tremendous mystery.
"Things could also go wrong if you misjudged the level of agony and pain. It was important to touch on things in this man's childhood that indicated he wasn't born with a silver spoon in his mouth, and yet we had to avoid it becoming a poor me or poor-little-rich-boy story. Self-pity would have killed us."
The King's Speech is due for release in the UK on Friday January 7.
Colin Firth: 'No experts on how to stammer'