Jake Gyllenhaal argues with his sister Maggie about changing his appearance for a role.

Jake Gyllenhaal

Jake Gyllenhaal

The 36-year-old actor underwent a gruelling workout regime prior to playing the role of middleweight boxing champion Billy Hope in the 2015 movie 'Southpaw', but his 39-year-old actress sibling insists he shouldn't change himself to suit his parts.

Speaking to the Guardian newspaper, the dark-haired hunk said: "My sister and I have the argument often. I've lost weight or learned a new skill and she's said: 'You don't have to change your body to play a character', and there are times where I agree and other times I don't."

And the 'Nightcrawler' star believes it is an added "craft" if an actor can transform their body when they take on the role of different characters.

He said: "I mean, putting the time into preparation, being agile, no, I don't think transforming your body physically had anything to do with the craft of acting.

"Although no matter how far you push yourself, to understand even the idea of that is to understand even the idea of that is almost close to impossible."

However, the 'Life' actor believes everyone thinks they have the ability to be a film star when it is a much more difficult job than it looks.

He said: "Because it seems to me that anybody feels they can be an actor nowadays.

"People say: 'How hard can it be?' But the truth is, the creation of a character, the understanding of human behaviour, is a craft and to be learned and worked and experimented with and discovered. My favourite actors are like artists."

Meanwhile Jake, who has previously dated actresses Kirsten Dunst and Reese Witherspoon and model Taylor Swift, has hinted he felt "constantly scrutinised" when he was in a relationship, and now prefers to keep his private life a secret.

He said: "I think when you're in a relationship, you are constantly scrutinised, your friends are scrutinised, but I would love to not talk about my personal life.

"Well, I have a firm belief that I enter space with an open heart and a mind that's strong enough to protect it, I'm not necessarily guarded, but I consider intimacy to be very important and I don't think everybody needs to know about my family or my personal details. I'm old school in that way."