But I’ve never seen harmonica-man talk with anyone on the bus.

That’s becuase he doesn’t need to. The bus is only a conveyance device. You and your fellow travelers are just occupying the same space in his journey - you weren’t invited along on the trek he’s making. He is uninhibited about carrying on conversations you’re not privy to in the same way that people pick their noses in the car while surrounded by windows on all sides. Somehow they think there is a cloak of invisibility around them. Curious phenomenon, that…