That has never happened to me, but it is very rude. What if you were expected to go to another function shortly after this one? You would either be late or cut this one where the rudies are shorter, and then they would expect to pay you less.
I always call ahead of time and double check the details (who, what, when, where, and how) but if there is something at the last minute to delay your performance, even a call won't help.
I suppose if you weren't in a hurry, and you have lots of patience and time ... maybe you could use that time to go to the bathroom. There is an unwritten law someplace that the minute you are in FULL CLOWN, Mother Nature will arrive on the scene and you will have to use the bathroom, but if you're like me, it's too much trouble, so I just postpone that. So ... if I had to wait awhile, I would probably find the nearest restroom to double check my makeup, costume, and use the facilities ... that way, when it is your time to perform, you will be in good shape.
Or ... you could use this time to bring in stuff, set things up for your gig, make a phone call from your car ... I don't know. I'm hypothetical here. It's never happened to me. I would make sure it didn't happen again if they called me back for a repeat performance by telling them at the time they call you that you DO have another gig that same day, and remind them that you charge by the hour from the time you arrive on the scene until you are finished. I'm sure you are charging close to $100 per hour ... believe me ... they will keep you on schedule.
Sometimes people take advantage of clowns because we are so nice. It kills me how many people expect a gratuitous gig ... I don't think they stop to think what goes into our performance, costumes, etc. That's a whole different subject though. Sorry ... I will now step down from the soap box, take a bow, smile and wave and make the little kids happy again.
