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.

Collected letters: 1926-1950 (ed. Vintage, 1988)