Dog only makes a few appearances in the game, but he definitely makes the most of them. The first is really a tutorial on how to use the gravity gun; Dog throws you some crates, which you try to catch, then he gets his ball (a deactivated rollermine), which you can throw around and make him fetch. It's fun to see him knuckling around the courtyard, chasing after his ball, and throwing it back to you (he's actually got a grav gun built into his hand).|
I really enjoyed the way Valve incorporated their tutorials right into the storyline. Starting with the metrocop ordering you to pick up the can he knocked over, to Dog teaching you how to deal with rollermines, their tutorials are fun, educational (fun-ucational!), and most importantly, don't drag you out of the moment.