I touched on this yesterday. First, you have to know what is difficult for a musician on the particular instrument you are writing for. You have to at least have an idea of the outer ranges of their ability, so you know what you’re getting into when you hand them your music to play.
For example, when Igor Stravinsky had The Rite of Spring premiered in 1912, the opening notes in the bassoon were usually not written for that instrument. From what I have heard, the idea was that it shouldn’t be so very pretty, or at least he couldn’t have expected that when he gave it to the orchestra. Bassoon players since that premiere have refined their skill, and often practice that solo for auditions, because if you can play that solo well, it means you have mastered the high range of the bassoon.
When I wrote Requiem last year, there is this part where the trumpet has really long notes. For a string player, a long note is not particularly difficult to play, but it is more challenging on the trumpet. Thankfully, Jason Bergman is a very skilled trumpet player, and he pulled it off very nicely.
I guess the trick with becoming really good at most things is to be able to make it look nearly effortless. So a piece can look and sound deceptively easy to play despite its difficulties, when you have a professional play it.
When I was working on the symphony, I was writing a sequence of notes for the contrabass that I hadn’t been writing before. It’s not that they were especially difficult, but I felt like I should show them to my contrabassist friend before settling on the bowings for them. Also, I know that bowings often get changed by the orchestra that is playing it. She reminded me of one thing that is difficult on the bass – multiple string crossings in rapid succession. Being a large instrument, and each note needing some time to start to resonate, really rapid notes can become muddled.
For a singer, I know a couple of things that make it difficult. Unusual interval skips can be challenging. Singing at the top of the register for an extended amount of time is also difficult, but in a different way. The unusual skips just means practice more to learn it. The extended high range tires out the voice, which shortens how long you can sing. I think these limitations are similar for most wind and brass players. From what I understand, a flute player might have a hard time getting it perfectly in tune as well, when you start hitting the max three or four notes in their register. Oh, and very long phrases have got to be divided. There is a limit to how long you can sing or play one note.
For a harp, having only seven pedals, which change the tuning for the entire harp, it is difficult to play lots of accidentals that are changing from one note to the next. You have to give them time to change the tuning. I haven’t tried to push it too much with my harp parts yet, just because it’s easier to write something that is “safe” than to extend what orchestras usually play.
And obviously, for all you piano players, it’s difficult for most pianists to play more than an octave per hand. Yes, many can stretch a ninth, but that is difficult and nothing to count on, especially for those with small hands.
When have you noticed that a piece you wrote was more challenging than you expected?