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Compare the overheard speech in these games to that in Oblivion and Mass Effect. In both of these games you can listen in on full-length public conversations. I would argue that this approach is simply less successful.
Those full conversations are, first of all, a bit "uncanny valley," since in reality the speakers would stop talking or turn to you as soon as they noticed you just standing there ... awkwardly listening in. (And Assassin's Creed does in fact feature that sort of reaction!)
Worse yet, they come while you're in the middle of gameplay (exploring, questing, or shopping), and they force an awkward decision on the player -- whether to stop and passively listen for a minute, or walk away feeling like you missed something. Neither option is great. This is verisimilitude versus realism in a nutshell: Full conversation dialog might be more accurate, but carefully tailored sound bites capture the essence of overheard speech far better.
On the flip side, those lines in Assassin's Creed suffer, as the game does across the board, from some serious lack of variation. Achieving a satisfying level of randomized variation in short, repeated dialog is a big challenge in games. More lines and more voices are expensive, but the payoff -- not annoying the player or jarring them out of immersion -- is pretty big.
In Dawn of War, getting six or more versions for common confirmation lines made all the difference. As far as writing variants goes, I am surprised every time at how subtle the difference can actually be, and work well. Even getting two unique good takes of the same line can be effective in avoiding the sense of repetition.
The key, of course, is to keep dialog short where it counts. And the hard part is in knowing when that is. Dialog that's in the environment, tied to gameplay mechanics, or that plays during game action really needs to stay short, clear, and direct. But that is never an excuse for lower standards of writing.
Very short dialog (under six seconds, averaging two) is critical for information that needs to be digested instantaneously. Merely short dialog (let's say as long as 15 seconds, but averaging closer to eight) has the flexibility of carrying a lot more information and character, but can't reliably be used while the player is fully engaged in intense, focused play.
Obviously, the pressure is off when you've got the player's attention and they are largely passive, such as in cinematics, dialog trees, and when they can safely listen to narration over their current task -- that is, for untimed puzzles and nonverbal, visually centered challenges (as in Portal, for example). Still -- I would argue that there are precious few cases where a single line of dialog should run over 20 or so seconds.
Valve Software's Portal
Here, then, is my list of cases for which short dialog should be used: unsolicited dialog, as heard in crowds, through peepholes, over the radio, from merchants, from comrades as you pass them by, and in the midst of combat; confirmation audio for player-issued commands; introduction and tutorial sequences of gameplay; and finally, I would argue, for brief midgame cut-scenes and plot-driven custom events.
Now out of that list, the tutorial is the one item that is not often all that short-winded. You could certainly argue that there have been plenty of successful, long-winded tutorials. However, a quick look at the tutorials in Fable, Assassin's Creed, Psychonauts (granted, it's spammed rapid-fire), and Rock Band show how well concise jots of dialog work for the purpose.
Determining when dialog and text in general should be short or long may be more important in video games than in many other types of writing. The player is not an audience -- they are not passive -- and we shouldn't be trying to verbally upstage them when it's not our turn.
I hope that, in singing its praises, I have done short dialog proper justice. It should be deft, on the nose if need be, full of character, and dutifully to the point. It should echo gently in the background when called for and give an ear-ringing shout when its turn comes around.
Short dialog should be there to call the shots in the heat of the action, and then step aside with a bare, curt nod when the letterbox descends and its longer cousin clears its throat and begins to speak.