One of the things I appreciate about O'Brian's writing is how he uses music and Jack's and Stephen's manner of playing as a reflection of their emotions, sometimes unclear to themselves.
He had boundless confidence in Stephen, but deep in his mind there was a sense of having been - not tricked, not quite manoeuvred: perhaps managed was the word. He did not care for it at all. It wounded him. He took up his fiddle, and standing there facing the open stern window and looking out on to the wake, he stroked a deep note from the G string and so played on, an improvisation that expressed what he felt as no words could have done. But when Stephen behind him, speaking over the sound, said, 'Forgive me, Jack: sometimes I am compelled to be devious. I do not do it from choice,' the music changed, ended in an abrupt, cheerful pizzicato, and he sat down again. (Desolation Island)
This passage comes to mind, wonderfully adhering to the "show, don't tell" principle while not only reflecting Jack's conflicted emotions in this particular situation, but also his personal take on their relationship as described previously.
I am sure there are many passages like this one. Which are the most notable examples in your eyes?