The pop heartthrob insists the band's boss has a softer side and often gives them helpful career advice and isn't ''untouchable'' despite his status in the music business.
He told Britain's Metro newspaper: ''He came to see our show in Vegas, which is good. We did feel very privileged because obviously he doesn't watch shows that often.
''He stayed behind to tell us what he thought and spoke to our parents and stuff - so it's not like he's an untouchable person. He's a good laugh and he's funny.''
The 20-year-old singer was surprised the millionaire music mogul didn't go gambling instead of watching the boys, but joked he needs to be more sensible with his money now he has a baby on the way with girlfriend Lauren Silverman.
Liam said: ''You'd normally be gambling in Vegas. If I was him I would have done but I suppose he has to look after the pennies with a kid on the way. He's got to take care of that.''
The group invited Simon and Lauren to join them on stage during their recent 1D Day seven-hour live stream, which was watched in 61 countries.
The novel's author saw a cut of the film and loved each of the changes the movie's director had made.