I think it depend of the feeling you want to give to the player !
And do you want to make a single soundtrack or more ?
For exemple I always make a soundtrack for each character (about ten now, since they change for each stories),but also for the "Investigation" part and for the "Answering Questions about the Case" part.
So for the characters, if it's someone who looks a bit stupid or joyful, I'll make a joyful song, with piano and flute. If it's a selfish/dark/mysterious character, I'll take violins and so on.
You can also mix all of this instruments, it can give something very nice !
I don't know if I answered very well, but well...
P.S : So yes, it really depends of the feelings you want to give
