From 2007:
An opera libretto resembles a play at first glance, but the differences are bigger than you might realize. To begin with, a libretto is shorter than a play, since it takes longer to sing words than to speak them. For the record, the first draft of The Letter is 8,700 words long, roughly half the length of the play on which it’s based. (By way of comparison, Macbeth, Shakespeare’s shortest tragedy, is 18,301 words long.) I expect it will shrink still further before it reaches its final form. Yet our version of The Letter tracks the action of Maugham’s original play fairly closely.
So what happened to the rest of the play? Some was cut outright—I started by eliminating two of Maugham’s characters—and the remainder was trimmed ruthlessly….
Read the whole thing here.