At my ballet school, we are blessed with having live music to accompany our classes, even in the leisure programs. This is the ultimate luxury, to me, because we don't have to find the right spot on the CD player, and exercises are not dependent on how long the music is. What this means, of course, is that the musician has to have incredible adaptive skills.
Teachers will set an exercise that isn't square (ie a tidy set of things done in multiples of four or eight bars that match standard classical music phrasing). Teachers won't know the exercise before-hand, so they figure it out as they go along and then have to suddenly invent additional bits or cut the music suddenly. Teachers will ask mid-way for music to speed up or slow down. They will set the exercise one way, counting it in fours and then suddenly realize it works better as a waltz or 6/8 and start over, with the musician expected to find new music in about 5 seconds - no exaggeration! On better days, we get through the exercise once but aren't bouncy enoughm, so the musician has to find something that will encourage us to jump, instead of slogging through to the old music.
Beyond the mechanical challenges, musicians need to know an incredible amount of repertoire. They seem to know all the important pieces from the standard ballets, plus an endless supply of classical, some folkloric, popular and even Disney. Last week, we did an exercise to "I Feel Pretty". Two weeks before, it was the incredibly sensuous tango from Scent of a Woman. I have danced to Chopin, Delibes, Tchaikovsiky, and the theme music from The Friendly Giant (Early One Morning, an English Folk Song). On top of this, our musicians write a lot of their own music. This week, our substitute pianist, Elena, played something quite lovely at the end of class. It turned out to be something she had written herself.
When I was a young music student, I thought I would like to be a professional accompanist for dance classes. I had seen pictures of such creatures, but had never met any. I'm not sure I believed they were real. Having gotten to know quite a few of them over the past eight years, I am convinced they are quite magical.