The mice coming out if you play the violin for them is a BRILLIANT idea, I'm definitely filing that one away!
Yes, my plan is that observing details of characters and crime scenes will open up dialogue options and further the story. It's tricky balancing that and not wanting to put the player to be put in a situation where they can't progress because they didn't happen to put their mouse over some tiny pixelated detail - so I think having an in-game hint system for what kinds of things you should be looking at is really important!