Speaking of musicals, again: orchestration has always been one of the aspects of musical theatre that I've been most interested in. I always used to look at the cast album to see who the orchestrator was, and formed a very vague idea of what different orchestrators' "sounds" were. I later found out that the idea was even more vague than I knew, since nearly all orchestrators had to use uncredited help, sometimes just for incidental music, but sometimes for major numbers. Steven Suskin's recent book "The Sound of Broadway Music" was the first to give a comprehensive idea of who orchestrated what on each show, which finally made it possible to discuss individual styles with any accuracy.
What the book doesn't fully explain -- it spends some time on that, but maybe not as much as I hoped -- is how much actual composing an orchestrator does. If you listen to a song on a demo recording, before the orchestration is done, and then listen to the final version, there's often a lot of musical material that the composer didn't put into the melody or the accompaniment. Bits of orchestral commentary between the stanzas, countermelodies, even quotations from other composers (the orchestrator of A Little Night Music, Jonathan Tunick, threw in a quote from Rosenkavalier as an in-joke near the end of the first act). Some of the additional musical ideas in these arrangements probably come from others -- the dance arranger, or the composer's assistant, and maybe sometimes even the composer himself. (Sometimes it's the conductor: on Li'l Abner the conductor, Lehman Engel, got a credit for "musical continuity" for writing everything except the actual tunes.) But at least part of an orchestrator's job seems to be adding to the song, rather than just arranging it as it appears in the rough piano version.
Here's a random example, just because a piano demo happens to exist. This song from Stephen Sondheim's Anyone Can Whistle is supposed to be sung by an uptight nurse pretending, unconvincingly, to be a French seductress and trying to seduce a mental patient who's pretending to be a doctor. It was that kind of show. I don't think much of the song except for one line: "I like your, 'ow you say, imperturbable perspicacity." Though that one line is so good it makes the whole song worthwhile.
And here's the song in the orchestrated version by Don Walker. Walker is probably my favorite Broadway orchestrator, even though many songs he's credited with were actually ghosted by others. (He took on so many shows in the '50s that he adopted a "factory" system, hiring multiple assistants and farming multiple numbers out to them. By the '60s, his assistants were off doing their own shows, and he reduced his workload.) He started with a reputation as a jazzy orchestrator, and mostly did musical comedies in the '40s and '50s, but what he loved best was lush, romantic shows that allowed him to conjure up a unique sound-world: among his favorite jobs were Carousel, Most Happy Fella and She Loves Me.
Anyone Can Whistle was Walker's only show for Sondheim, and he apparently thought that the score was needlessly complicated. The former Walker assistants who orchestrated Funny Thing Happened On the Way To the Forum felt the same way: Irwin Kostal is quoted in The Sound of Broadway Music that Sondheim nearly ruined "Love, I Hear" by giving him a sketch with pointlessly complex accompaniment and insisting that all his accompaniment ideas be used in the orchestration. (Luckily, Leonard Bernstein heard the arrangement during previews and bawled Sondheim out for it, shaming him into changing it: "Who do you think you are? Me?") By the time Sondheim emerged as a successful composer with Company he had improved a lot as a composer, or at least figured out how to make his clever ideas feel organic to the song. His later success as a composer naturally reflects back on his early work, but I don't think Anyone Can Whistle is as good a score as many that were being written in the early '60s.
In the case of the Whistle number, anyway, Walker and the dance arranger (who I'm assuming is responsible for some of the ideas in the dance breaks at least) don't change much about the music. It's more a question of trying to liven it up and support two performers who aren't really singers (Lee Remick and Harry Guardino), finding different "fills" and decorations for each stanza, to give each repetition the feeling of being somehow different in color and tone from the one that came before it. The overall effect is to make Sondheim's music sound more conventionally Broadway, but that's what this number needs to make it work.
The other thing about Walker's work here is how he pulls off a very strange choice of instrumentation. Forum was one of a few shows without violins, but Walker here went his trainees one better and also cut out violas -- the string section consists of five cellos, plus a bass. He handles this so well that you hardly notice the absence of higher string instruments, but it gives a subconscious dark, creepy feel to the whole show.