I was trying to round out a character I have in mind for a future project, and making the usual decisions about what sort of details to include. I'm not talking about height or hair color or those sort of physical descriptors, but the little scene/character builders that often only get a line or two mention in a story.
I'm not out to do this in in great detail, mind you. I've seen (and have, somewhere on my computer) those lists where you're supposed to come up with everything about your character. What kind of music they like, what books they read, the name of their first kiss, pet, and/or houseplant, that sort of stuff. I don't buy into those too much because I think some of it is just a distraction, and worse, if you write a serial character, you take away some of the process of discovery. If I have a character who's going to be around for several books, I expect to learn about him or her along the way. Otherwise what's the point?
Two examples come to mind. The first is a classic, and it's Sherlock Holmes and his violin. Everyone who's ever read a Sherlock Holmes story or two knows about the violin. (And probably the cocaine addiction, but that's another story.) To the best of my knowledge, and while I have read them all it has been some time so I may well be wrong, but as near as I remember, not once did that particular skill ever help Holmes solve a mystery. Yet so iconic is it that it even made it into that "Young Sherlock Holmes" movie, albeit as a brief, almost throw-away moment in the story.
The second is far more contemporary. I won't name the show, but it was on in the 1980's and the main character had a cabin up somewhere in Canada or the Rockies or something. That bit has of course almost become a cliche for the lone hero motif, but what wasn't was the occasional shot of him on his front deck, playing the cello. I have no idea who thought of that, or whether the actor could even play, but it was a neat moment and it spoke volumes about the character.
In both cases the choice of instrument was probably dictated as much by the needs of the story and the medium as it was of the times. Other than the violin, there were few other instruments you could conceive of the Great Detective playing, and few others that would fit in the apartment at 22B Baker Street (the grand piano was definitely out, though I could see Holmes as a pianist). As for the cellist, I suspect someone decided it made for some good cinematography, and was something the lead could play while looking good. The harmonica can make you look kind of silly, for example, even when you're really good at it.
I thought of something similar to do with my character, and my reasons for choosing the instrument I did had as much to do with the character as they did with the particular scene I have in my head, and how that's going to play out.