Before a clown goes on stage, he must first of all change his appearance. This usually involves changing clothes and applying makeup. When he goes on stage, the audience will laugh at the clown because he looks funny.
When the clown is on stage, the audience might also laugh at the clown because of his performance. For example he might walk in a funny way, or he might do funny things, like throw custard pies at people.Notice also that appearance and performance have different meanings in their countable and uncountable forms:
You can see that in their countable forms, appearance and performance generally have the same meaning. However, you need to be careful with the uncountable forms of appearance and performance!
I'll give you a text that features these different meanings. For each example, can you guess which meaning I'm using? (a-d)
Clowns are not usually interested in the performance (1) of cars because that's not funny. Instead they ride unicycles as part of their on-stage performances (2). They also change their physical appearance (3) before they go on stage to make sure they look funny. A travelling clown makes up to 100 appearances (4) a year in different locations.