if the world is a stage and we all are actors,then who is the audience,

We are the audience too.
Even in movies, not all actors act at the same time. Some sit and watch.

and some stand and watch.

of course, we are ALL actors. Dont you know that some people's role is to be the audience?And in life, at one time or the other, we all are on the sidelines, watching, being the Audience., but dont spend too much time being the audience, be actively involved

God. As hes always watching

Regardless of the fact that some people may live their lives as spectators,God is the ultimate audience, and the devil of course!! depending on whose script you perform,you get a standing ovation and an award!!