With some time to kill recently, I browsed through my movies folder and looked for a few small things to watch. I came across the Halo 2 announcement trailer and the Halo 2 Realtime Demo.
Having watched the latter, I was especially struck by some of the themes I explored when comparing Halo 2 and Half-Life 2, as well as some of the contrasts between Halo 2 and Halo 1 with regard to the use of cutscenes versus interactive portions of gameplay for the purposes of relating story.
Much was made of the fact that the realtime demo was just that: a portion of a level played out by Bungie staffers doing a live demonstration, bookended with two cutscenes. The opening cutscene shows a Pelican carrying Sergeant Johnson and the Master Chief down to New Mombasa and landing; in the analogous sequence in the game, that Pelican crashes. The cutscene ends when the Pelican lands and the Master Chief disembarks; the change in the screen's aspect ratio signals this change.
However, the non-combat interactivity doesn't stop there. The Master Chief passes medics assisting injured marines and overhears their comments, and observes as Cortana, through the Chief, interacts with a corporal presiding over the death of the lieutenant at the scene, who then directs the chief to the new officer in command, Sergeant Banks. Sergeant Banks greets you, and then we see him call in an airstrike on a nearby Covenant artillery piece that has Banks' men pinned down. As we watch, three Longswords swoop in overhead and take out the artillery.
All this occurs during normal gameplay. As with nearly all such sequences in Half-Life 2, the player could have wandered away and missed part or even all of it. Valve seemed unafraid of this possibility, as they included virtually no cutscenes in Half-Life 2 at all.