If you go on stage with the wrong attitude, or something in your performance is off, you can lose an audience in the first minute. That first minute is crucial.
I was at college doing performing arts, and just spending all my time mucking about, and the lecturers thought I would be pretty good at stand-up, so I gave it a whirl.
I went to Montreal. My first gig went very badly. They just weren't laughing at anything. I found out they were a load of Christians, and it was a gig to raise money for a new church roof.