Bruce Willis has compared making the 'Die Hard' movies to childbirth.The 52-year-old actor - who reprises his role as maverick cop John McClanef or 'Die Hard 4.0' - says, like women and childbirth, if he remembered the physical pain he went through to make each movie he would never agree to do another one. Bruce told BANG Showbiz: "There is a thing they say about women and childbirth - they say if women remembered the pain of childbirth there would be no more children born, everybody would only have one kid. But, as time goes by you forget the pain, and I forgot that I was hurt, cranky and irritable."The star - who dressed in a black suit for the other nights glitzy London premiere - also revealed he had to step up his fitness regime to survive shooting. He said: "I started working out three times a week and I thought that would be enough. But then I started jumping on refrigerators, diving on the floor and knocking my head on steel, and I realised I had to work out more to get my muscles stronger so my bones wouldn't shatter on the concrete. I tell you, I used to bounce of the concert a lot easier than I do now."I'm a 52-year-old man!"Bruce's co-stars Maggie Q, Timothy Olyphant and Justin Long joined the action star on the red carpet, along with director Len Wiseman, who was accompanied by his glamorous wife Kate Beckinsale in a tight-fitting white dress.