Even if you're playing the most well-known repertoire under the sun, I still believe you have a responsibility as an artist to tell the audience why you're playing it, what are the key aspects to it, and then throw in a bit about its historical context.
There's always blood on the carpet when I play Beethoven at the piano. I hate playing the piano! And it's so hard to fight for Beethoven's soul! But that's what I have to do!