The first time I was on stage, this is how it happened. I worked in a bar that had live entertainment, and got to know one of the regular acts pretty well. One night the guitar player yells my name, handed me his guitar, and said, "Wildwood Flower" and disappeared. I got through it just fine, mostly I think, because I didn't have time to get nervous.
What a great feeling.
Yet another hobby that is completely out of control...