Someone playing a 3rd Person Shooter or FPS doesn't have the same level of peripheral vision and environmental awareness that they would have in the real world. Special events therefore may go unnoticed, simply because the player was looking in the wrong direction at the time. In Gears of War, the camera can be focused on a special event at the press of a button. The first indication that something special is happening and worth seeing is therefore often a giant blinking controller button prompt. It's functional to be sure, but requires the player remove themselves from the action of the game for a moment in order to view the event from another perspective.
In a game like Half-Life 2, the player is not even allowed to relinquish camera control voluntarily. Maintaining the first person perspective and player interaction at all times means "Press X to look at cool shit!" is not a viable way to get the player's attention. The player's attention has to be grabbed without "external" interference, such as a GUI prompt or camera movement not tied directly to the mouse. One of Valve's favourite ways to do this is with birds. When they want the player to look in a certain direction, they have a flock of birds scripted to be startled and fly off, drawing the eye. Though subtle and brilliant, it loses its effectiveness when used too much. The problem is that birds always fly towards something the player is obviously meant to look at. A trick using the natural ambient life of a game world to attract the player's attention then just becomes a tool to attract the player's attention. Birds are never just birds in HL2. Their function as beacons completely overwhelms and replaces their value as ambient life. Like Neo seeing code instead of his artificial reality, a player enlightened by the developer commentaries won't see birds at all, just a script indicating that the player is supposed to look over there now. And once you've figured out the purpose of the birds, it becomes glaringly obvious when some other object in the game world exists only to attract your attention.
Sometimes the architecture of the level can be used to draw the eye. The best example of this has to be when you exit the train station at the beginning of HL2. They wanted every player to notice the giant Citadel in the distance, but it wasn't noticeable at eye level - the CItadel is a few kilometers off in the distance, and 4-5 story tall buildings across the plaza obscure it at eye level. The player would have to look up into the sky to see it, and there was no reason to do so with so much activity occuring on ground level. To get players to look up they placed an obelisk in the centre of the plaza with some screens displaying propaganda fixed to the top of it. Upon exiting the train station, the player will hear the propaganda announcements and look up to better view the source. At this point the massive Citadel comes into full view, dwarfing the obelisk in a brilliant juxtaposition. The player feels like they "found" the Citadel on their own when in fact the entire plaza had been designed in such a way as to present the best possible reveal of the game's literal centrepiece and the player's ultimate destination. And yes, there's even a flock of birds that flies off towards the Citadel.
Whether it's architecture, NPCs reacting to something "off-screen," or objects included for the express purpose of attracting the player's attention, using things that exist in the game world is the best way to make players turn their heads of their own presumed free will. Not only does it maintain immersion, it's more true to the interactive nature of the medium. Sure, a designer can cross and jump-cut as he rips control from the player's hands in order to display his cinematic prowess, but that's ignoring the fundamental potential of the medium. Video games require new techniques and a new language for setting the player's viewpoint. With the player in control of the camera, it's the designer's job to create circumstances that encourage and suggest, not force, ways to view the game world.