Morkus from Orkus
Command is not an area of effect spell, though, so you can't just toss the spell out and catch the guy behind the boulder in the spell effect like you would a fireball. Even if the guy can hear the words being spoken, the magic still needs to be directed specifically at him. An "I can see the guy standing there, so I'm laying the magical compulsion on his person that will be triggered by the words that I speak." sort of thing. If you can't see him, even if he's the only one behind the boulder, you can't see where to place the compulsion magic in order for the command you speak to have effect.OK, that's fair.
But when there's nobody else around except the one guy hiding behind a boulder, and the caster can see the boulder and thus knows where that person is, I'd think a Command should work.