'Schauspieler' is rather a term for a profession/job.
A 'Darsteller' doesn't necessary need to be a 'Schauspieler'. For example a play in primary school. You could call the children who are playing their roles 'Darsteller'...but they are not really 'Schauspieler' yet.
But the other way around it works better. Let's say you've watched a movie and in the credits you may read 'Darsteller'.
Ufff...not easy to explain ^^* I hope you got an idea of what the difference is.