Sometimes we go to a play and after the curtain has been up five minutes we have a sense of being able to settle back in the arms of the playwright. Instinctively we know that the playwright knows his business.
For me, filmmaking combines everything. That's the reason I've made cinema my life's work. In films, painting and literature, theatre and music come together. But a film is still a film.
When you want to put something into your part that is not in the play, you must ask the author-or some other author-to lead up to the interpolation for you. Never forget that the effect of a line may depend not on its delivery, but on something said earlier in the play, either by somebody else or by yourself, and that if you change it, it may be necessary to change the whole first act as well.