Christie described her training for the role of sword-wielding skilled warrior as "intense" and "painful".
"About three months before filming I have to heave myself out of bed and meet a personal trainer," said Christie at the UK launch of season three.
"I train three, four, five times a week, protein six times a day, resistance training for at least 45 minutes... it's so very boring. It's really painful. It's laborious. I can't go boozing. It's very intense."
However, Christie said that the physical effort was worth it and confessed that she had argued with her own agent about taking on the role.
"I read the books and fell in love totally with the character. She was doing everything I want to do as an actor. She was playing with gender stereotypes, she was a very strong woman and she was outside of what we normally deem to be attractive and she was outside the usual rules of what make women powerful, which are created by heterosexual men," she said.
"I immediately rang my agent and said, 'I want to do this'. My agent said, 'What are you taking about? I'd never ever put you up for this. She's ugly, her nose is broken, her teeth are broken and you'll need to use a sword'."
Christie, who is 6'3" tall, revealed that she had suffered abuse since she was 14 years old about her height, and said that she hoped the character of Brienne could "challenge some prejudices".
"To be able to take this issue into a forum through my art and maybe even change just one person's mindset would actually justify my existence as an artist," she said.
Game of Thrones returns for its third season on Sunday, March 31 on HBO in the US. It premieres in the UK on April 1 on Sky Atlantic.
Watch Game of Thrones stars Rose Leslie and Sophie Turner discuss season three below:
Game of Thrones season three - pictures: