Billy Crystal's grandparents were ''not fun''.

The 65-year-old actor remembers his elders as being less than jovial, which has spurred him to be far more playful around his four grandkids, who are all under ten.

Billy told Metro newspaper: ''When I think of them [his grandparents] I always think of them as old.

''Even in their sixties they looked like they were in their eighties. They were not fun.

''I've already had more fun with my grandchildren than my grandparents ever had with me.''

Despite their lack of cheer, Billy said his grandparents taught him a lot about life.

He said: ''They were really interesting people. You actually did feel like you were going to glean real wisdom from them. I remember them with great respect and affection.''

Billy has previously said he his grandchildren demand he talk in the voice of his 'Monsters, Inc.' character Mike Wazowski.

He said: ''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.''