Interestingly, he's working on a book of his complete collected lyrics (including alternate, cut lyrics, etc), with introductory essays and commentary on the lyrics for each show. He said he just finished the chapter on Sunday in the Park with George and is about to start Into the Woods.
When asked about his favorite piece that the audience might not know well, he mentioned "Multitudes of Amys", which is a nice number that was cut from Company. I like it a lot, so I was pleased with that suggestion. I want to go home and sing it again tonight. There aren't many recordings; he mentioned on on a record called "Symphonic Sondheim". I thought it was an optimistic song, but he said it was actually about the main character in Company convincing himself that he should get married to a girl who he actually doesn't really love enough. Sort of sad. I didn't hear the lyrics that way at all, so I'm a bit confused. He also said in some other interview that "No One is Alone" in Into the Woods isn't really saying "you won't be alone", but rather "everyone's actions affect everyone else". Sort of a weird interpretation to me, but hey, he wrote it!
On the question of which come first -- lyrics or music, he said both come together. But he also said that the lyrics suggest a musical contour. He mentioned that Cole Porter used lyrics to suggest a rhythm -- (gotta use that reference in my paper on my rhythm-suggesting algorithm!!). Sondheim, if I understood correctly, does that too, but also uses the "inflection" of the phrase to suggest musical ideas. His example: "Finishing the HAT". with emphasis on HAT, and a sort of rise in pitch as the phrase progresses. This short discussion was the most detailed piece of info on his creative process I got out of the evening.
I liked his story about the afternoon of taking criticism on his first musical from Hammerstein. I'd heard it before, but it was great to hear him tell it. In particular, he said it was like getting the whole of Hammerstein's career full of experience, distilled down into an afternoon of the most important principles of writing musicals... but not only that, he was 15 years old or so, so his mind was like a sponge, and internalized everything. He said that he still uses those exact lessons when he writes today, and that everything he learned he learned from Hammerstein.
This makes me really want to have the experience of Sondheim giving his distilled lessons in an intense afternoon! I guess I'd have to be 15... but it would then be like getting both Sondheim and Hammerstein's careers in a super-compact form! Taken to the extreme, this reminds me of the Bene Gesserit women from the Dune series -- they each seem to be channellng the memory of their ancestors, so each new person in the line gets all the former lifetimes of experience in addition to her own new ones. Of course this goes wrong in Evil Alia, but anyway it's neat to see it in real-life -- not in sci-fi, but in the near-sci-fi phenomenon of Stephen Sondheim's awesomeness!
I suppose I feel a little like this when I learn things from my advisor and from reading the Ph.D. dissertations of his former students -- note how the end of the previous paragraph, for instance, came from one particular day of DRH editing a few pages of my writing a few years ago. That "-- not in sci-fi, but in the near...." is a construction he would use.