Atonal, almost a-metric: I had no chance to listen to actors and take timing before hammering out nearly forty-five minutes of underscore. So I wrote a lot of dead-simple ostinato and whole-tone stuff that could start and stop and would never be clearly "hey, there's an entrance here in the underscore but the actor hasn't gotten there yet."
(I also had only a week to write a whole play's worth -- with very little input from the director. This was "Agamemnon," a somewhat too modern translation from Aeschylus, and the direction given was "sorta post-apocalyptic." Thus lots of sound beds of ancient instruments, industrial sounds, and synth pads.)
Constructed with whatever I had on the laptop; freeware VST synths like "Da Hornet," sound effects and Pro Samples instrument CD's programmed into VSamp, the piano and Halion stuff packaged with Cubase. Mixed the thing down in stems and ran the show non-linearly on Qlab (http://figure53.com/qlab/), meaning I could drop out a alpenhorn and marching Romans line and bring in Ms Irina Brochin when the King stopped talking and Iphigenia came in. More or less.
All in all, I'd say the process was more interesting than the resulting music. But it was fun!