Gwyneth Paltrow thinks her Iron Man 3 co-star Robert Downey Jr. is a ''wimp''.

The 40-year-old actress, who is married to Coldplay frontman, Chris Martin, explained that she couldn't understand why Downey was ''in hell'' every time he donned the superhero suit for the action movie.

Speaking to Graham Norton, Paltrow said: ''I get to do a bit of action and I get to wear the Iron Man suit for a while - it's very cool.

''It's quite cumbersome but not so bad. I thought it was going to be terrible because in the first two movies Robert would put on the suit and start sweating, he'd have a nervous breakdown and would meditate and cry. He was in hell.

''So, I was like, 'Wow, I'd better start mentally preparing myself for the suit,' and I put it on and was like, 'This guy is a wimp, it's really not that bad.' ''

Paltrow, who has children Apple, eight, and Moses, seven with Chris, further explained that her children were extremely impressed by her role as Pepper Potts.

She added: ''I'll never forget his face [Moses] when he saw me walk out in the suit. It was a really good moment.'

''I love watching his seven-year-old brain put together who I am. We were at a Justin Bieber concert and Moses said, 'Maybe Justin knows who you are from Iron Man."