I read recently in Center for Contemporary Opera's Opera Today (Fall 2009) an interview with J. D. McClatchy, author of many a libretto. He was asked the question of how he decides what subjects can become libretti, or as the interviewer put it, "stage-worthy." His response:
My own experience—of both watching and making operas over the years—tells me that melodrama works best. Comedy is rarely funny, and its impulses are handicapped by the slowness of music; tragedy that begins with too abstract or merely psychological a premise grows wispy and tedious. But melodrama offers the possibilities of variety, outsized characters, and a plot that is both complicated and resolved.
I found this a surprising response but not because of any silly notions that there might a "right" or "wrong" answer to such a question. Rather, because it was so precise and because I had no idea how I would have answered the same question. I'm also surprised that I haven't been forced to formulate an answer before.
Each librettist will have different ideas and you'll have as many answers to that question as you ask librettists. So here I'll answer the question for the first time.
My short answer is "anything—with enough effort."
The more considered answer goes something like this. Inspirations come for all corners: news stories, biographies, poems, novels, plays, television, daydreams. While with enough effort, anything can be crafted into a libretto, some ideas provide a better and more immediate payoff to this effort. To get an idea how much effort will be required, I consider these three factors.
Is there something, anything, that one might call intense about the idea, in particular, emotionally intense? This is not quite McClatchy's melodrama, as I feel the intensity can be an internal state as much as external conflict.
How easily can the story, events, characters, ideas be broken down into the component parts? Can you give the audience both something to take in at a glance and leave some room for subtlety, nuance and ambiguity?
Does the idea make sense outside its own time and place? This is especially important when considering historical subjects—how much exposition and background will the idea require to get across?
And now, I invite you, dear reader, to offer your answer—whether a librettist or not.