Creating an immersive game environment is as much magic as science. Detailed graphics and atmospheric lighting are only part of the formula; in order to provide a tangible sense of realism, the game world also needs to react to player actions in a convincing manner.
For Killzone 2's theater of war, the design brief goes one step further: the environment shouldn't just react to player actions, it should do so dramatically. To some extent, this form of 'Hollywood realism' can be achieved by adding small-but-impressive details, such as bullet holes glowing red-hot from the impact of depleted uranium shells, or bits of plaster and concrete chipping off a wall when subjected to gunfire.
But sometimes, bigger really is better, and more really means more. A spectacular setpiece event, when initiated through player actions, can help strengthen the player's sense of control over the theater of war. It also serves as a reminder that the game environment is not a static, unchanging backdrop to the action, but very much a part of the action itself.