ROBERT De Niro in Raging Bull. Christian Bale in The Machinist. Extreme body transformations tend to be regarded as a dramatic actor's domain.

But Chris Pratt, who gained 30kg to play Vince Vaughn's sidekick in Delivery Man, takes his comedy seriously.

"It really helped me in my performance because I felt depressed, I felt mentally dull,'' says the rising star, soon to be seen as Star-Lord in Marvel's new superhero fantasy Guardians of the Galaxy.

"I have a lot of heavy people in my family, I understand the emotional toll you pay when you are not good to yourself, when you are eating emotionally, when you are eating to hide your emotions and you are eating out of this sense of stagnancy,'' he says.

Clearly, writer-director Ken Scott's remake of his own French-Canadian film Starbuck is not your average Vince Vaughn vehicle despite the outlandish plot about an affable underachiever who finds out that his youthful sperm donations to a fertility clinic have fathered 533 children.

"It was really important to both Vince and I to stick exactly to the screenplay, not to ad lib, to really play the dramatic weight of the scenes and allow the laughs to come naturally," says Pratt, best known as the dim-but-lovable Andy Dwyer in NBC's Parks & Recreation.

He believes that the weight he gained lent an extra dimension to his character, a stay-at-home Dad who has put his legal career on hold to look after his four children.

"Also, there is a physical comedy that comes naturally from a person who is bigger, so there were moments I got laughs without having to do anything, just by wearing, say, a tight overcoat."

Pratt admits that his metamorphosis would have been much less attention-grabbing, however, had he not then lost every one of those 30kg and five more to play Peter Quill/Star-Lord in Guardians of the Galaxy, with Bradley Cooper and Zoe Saldana.

"I am really glad I have lost the weight since so it can be a choice,'' he says.

"If I was 300 pounds right now and I told people I had gained weight for Delivery Man they would be like yeah: that was a year ago, you are just fat."

BULKED-UP MAN OF STEEL PUTS PREVIOUS SUPERHEROES TO SHAME

Marvel has made a point of avoiding the obvious in translating its comic book characters to the big screen. Robert Downey Jr might own the role now, but he was a surprise choice for Iron Man when he was originally cast.

And Chris Hemsworth was a relative unknown before Kenneth Branagh (another unlikely convert to the Marvel Universe) directed him in Thor.

Pratt continues in that tradition.

"James Gunn believed I was that guy - I just didn't look like him yet,'' says the actor who auditioned for the role straight off the back of Delivery Man.

"I was still about 280 pounds. They said: be honest with us. Do you think you can get in shape? I said I know I can."

Pratt believes an actor's appearance accounts for one third of what he can give to a performance. What he sounds like contributes another third. And then there's "the rhythm of his spirit."

"I felt like my sound and my spirit was there. I had five months to work on the body."

And work on it Pratt did - as the photograph he tweeted to show off that hard-earned six-pack clearly attests.

Pratt, who has a son, Jack, with wife Anna Farris, credits his brother with that piece of inspiration.

"He probably should be my manager. He's a blue collar guy, a cop in Oakland, but he follows my career closely. He said: 'you got in good shape for this movie. No one saw it because you are pretty much wearing long sleeves and long pants and a helmet the whole time. People need to see this side of you, to see you are capable of doing that'."

Describing his body as just another prop, Pratt credits his physical transformation with opening up a whole new range of career possibilities.

"I am now in contention for roles no one would have considered me for before," says Pratt, confirming that he is in final negotiations to star in sci-fi reboot Jurassic World.

"I know I want to do it and I know the director (Colin Trevorrow) wants me to do it so now it's just a matter of everyone coming together and making sure it can work."

Delivery Man opens today