A lot of discussion on Gamasutra lately has focussed on the idea of player choice, and what "meaningful" player choices mean. Most of the discussion has centred around the idea of morality, something which is becoming more and more prominent in games. I threw in my two cents to that debate when the Gamasutra blogs first started up.
As interesting as morality is as a game mechanic, though, there is another potential mechanic that I think is frequently overlooked which could be used to great effect - time. We frequently discuss choices in terms of their moral consequences, and that's definitely an important aspect of choice, but not all choices have clear moral consequences. On the other hand, virtually every decision we make has an effect on the amount of time we have available to us, and that's one of the major factors that we take into account when making decisions.
Games do not frequently use time to emphasise the decisions we make, however. Timers in games are not generally used to give meaning to player actions, but to cause players to hone their skills to a specified point. If anything, timers present the opposite of meaningfulness by requiring players to do the same things repeatedly.
That's not to say that all games suffer from this problem. Real-time strategy games very much emphasise the role of time in the decision making process. Take too long to decide and your opponent will over-run you. Make the wrong build decision and you'll find that your defences fall apart.
In RTS games, though, there is generally a mathematically optimal decision to make. This decision may depend on what your opponent has done, but there will still be a mathematically correct course of action, and so a player is not necessarily choosing so much as solving a math problem. I've discussed why I think that can be problematic in the past, so I'll avoid doing it here.
Games That Use Time Effectively
It's easy to be critical, so I'd like to look at some instances in games where time has been used effectively. Just a warning, there will be spoilers within, so if you haven't completed these games, you may want to skip this section.
The first game that comes to mind is Indigo Prophecy. Many of the scenes in the game are time-limited, though the player is rarely told precisely how much time they have. This provides a strong sense of tension in many of the games scenes. If I know the police could show up at any time, but I don't know what time specifically, my fear feels much more organic than if I was simply trying to beat a timer.
The game also uses time well in its dialogue sequences, where the player only has a few seconds to decide how to respond or what question to ask. This more closely replicates the flow of an actual conversation, where most responses are reasonably quick. If the player does not decide quickly enough, the game automatically directs the conversation in a direction which will provide them with all the details necessary to keep playing, but they'll miss interesting side details. This strikes a great balance between keeping the game flowing while never allowing the player to get stuck.
On the whole, Mass Effect runs into some of the problems I've mentioned. Players can take virtually unlimited time to make dialogue decisions, greatly disrupting the excellent cinematic flow that the game has at its best. One particular decision in the game takes excellent advantage of time, however.
That decision is found in the latter part of the game, where the player's team has become split up and two potential team members are in danger, but are far apart. The player has to decide which one to save. The decision is not moral - there is no right or wrong person to save (though if you're trying to optimise your performance, there may be a mathematically correct choice to keep your party balanced). The limitations on the decision are purely time-based; the player can only save one team-mate because other things will be happenning concurrently.
I have attempted to address some of these issues in my own game Time Flows, But Does Not Return, which is very much about the role of time in the decision making process. I'd like to think that I've used time effectively as a game mechanic, but since the game is deliberately abstract it's difficult to draw any lessons for larger, more complex games.
Ways To Make Time Meaningful
There are two key lessons that I think can be drawn from the examples above. The first is that time-limited choices are best when the player can not make a "failing" decision. In Indigo Prophecy it is impossible to miss conversational details which are vital to progressing the game, while in Mass Effect no matter which character you save you are still able to complete the game.
Limiting the future options available to the player on account of time-limited decisions can work well, it may even be ideal, so long as there is always one option available to the player to move the game forward. Choices should alter the story, not end it.
The other lesson to draw is that other things should be happenning in the game world simultaneously. If the player makes choice A, choice B disappears precisely because they are happenning concurrently. Choice B should never disappear "because otherwise the player can get both awesome swords" or something equally arbitrary. This makes the setting and the story feel more organic - the player is part of the world, but they aren't the whole thing.
There are, of course, other games which have used time as a central mechanic. Braid deliberately subverts many of the things I've been discussing, for example, and while I haven't played it, my understanding is that The Last Express takes place in real time to great effect. Feel free to add other examples of games which have used time as a decision making mechanism in the comments below if you can think of any that I haven't.