Billy Crystal was ''driven crazy'' by his grandchildren making him do the voice of his 'Monsters, Inc.' character.

The actor provided the voice of the one-eyed green creature Mike Wazowski in the Disney animated film and his role impressed his daughter, 'House' actress Jennifer Crystal Foley's two girls - who are both under 10 - so much they wouldn't let him talk normally.

He told UK TV show 'Lorraine': ''My grandkids, when they started to understand what I did, because they didn't know, the first thing they saw was 'Monsters, Inc.' so then for six months I had to talk like Mike Wazowski. It drove me crazy.''

Billy, 64, also revealed he got the idea for his latest movie, 'Parental Guidance', from a lengthy babysitting experience he and his wife Janice Goldfinger had with the grandkids.

He explained: ''The movie started because we had the girls for six days. Alone. And on the seventh day I rested, then came into the office, and said, 'Alright, here's a movie. I'm exhausted.' Two grandparents babysitting for the kids while their daughter goes away, it's about old school and new school, how do you raise the kids, do you follow the rules or not? Do you just do it your way? And that's how it started.''

Billy - who reprises his role as Mike Wazowski in prequel 'Monsters University' - is also set to become a grandfather again, as he and Janice's other daughter, Lindsay, is pregnant with her second child, who will join her two-year-old son in the family's brood.

He added: ''I am the grandfather of three and two thirds - the new one is coming in March.''