hm well i did this text last semester, so this is all coming from my memory and according to the dictionary idealism means the cherishing or pursuit of high or noble principles, purposes, goals ...
so you could talk about Lewis - how he wants to be a better director (and a fair one esp. compared to Nick) and how this drives him to success with the play.
Roy - his dream of being the star in Cosi Fan Tutte as it was originally his idea of doing the play. In the end, he actually does become one.
Lucy/Nick - wants to change the world, stop the Vietnam war etc. kind of makes them too radical and disliked by Lewis and then eventually by the audience as well.
But with the other factors that helped them you could also discuss how they're trapped in the mental asylum and can't really explore things outside the building/develop that confidence. Or how they're discriminated by society so it gives them bad self-esteem, like Henry i think.