If you want to see a comedian who can go on stage with no material and still destroy, check out Patrice O'Neal. He was the comedian who other comedians were afraid of. He was good friends with Bill Burr. Bill talks about how comedians would be hanging out at a table ripping on each other, and as soon as Patrice entered the club, they'd stiffen up because they knew Patrice would sit down and start ripping on everybody.
Also, check out the 2002 film called Comedian. It's a documentary created by Jerry Seinfeld where he talks about all the preparation that goes into becoming a comedian.