The bald actor is not wearing a wig for his role as cop John McClane in the fourth Die Hard movie, Live Free or Die Hard, because he wants to show that the character has aged.
Willis, 51, has issued a stern warning to anyone who is thinking about ridiculing his lack of locks.
He said: "I'm a man and I will beat up anybody who tries to tell me that I am not a man just because my hair is thinning."
Willis is currently shooting the action movie in Baltimore, in the US.
It is scheduled for release next summer.
Meanwhile, Willis has revealed he wants to adopt.
The actor - who has three biological daughters with ex-wife Demi Moore - admits he would love to give a home a disadvantaged child but he doesn't want to do it on his own.
He recently said: "I would need somebody else in my life. I wouldn't want to just have a child and a nanny."