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Your development team should already own a copy of the sport's official rule book; if it doesn't have one, buy one. As I said, many of the events that can occur in a game won't necessary happen in the ones you transcribe. It's your job to have commentary for every event that can possibly happen within your video game. Fortunately, the number of these is somewhat smaller than the number of events that can happen in a real game. You don't need any comments about a fan interfering with the game, for example, unless for some reason you implement such an event. Here's a little flow chart showing the possible outcomes of a corner kick in soccer:
The programmers will have to implement code for each of these circumstances, and you will need appropriate commentary as well. The chances of a kicker in a corner kick handling the ball inside the penalty area before anyone else touches it is pretty remote, but you can bet that your players will try to make it happen if they can. If they succeed, your commentators should have something to say about it.
Sit down with the game's designers and programmers and consult with them about how much of the rule book will be implemented in the game. Rule books are full of truly arcane rules for circumstances that simply can't happen in a video game. For example, if a base coach deliberately interferes with a thrown ball in baseball, the runner is out. Many baseball games don't include base coaches at all; in those that do, the players have no way of controlling them, so there's no way to intentionally interfere. The base coaches are just cosmetic and could even be transparent to baseballs so as not to interfere with them.
Finally, don't count on the rule book for everything. You'll need to write commentary for events that the rules don't cover, such as naming the players as they come out on the field at the beginning of the game. This is why you need both a transcript of a real game and the rule book to tell you what you'll need to write.
At this point, you know the scope of your project -- whether you're doing radio-style or TV-style commentary, and so on. Using any prior script that may exist, your transcript of a real game, and the rule book, you should now work out a list of events that you want to write commentary for. Verify with the programmers that they are planning to implement all these events and can detect when they occur in the game.
You will need to create events both for the play-by-play and the color commentary. The play-by-play events will naturally come from the actual activity in the game, while the color commentary will apply both to particularly noteworthy events and to the failure or success of the player's strategy and tactics. For example, you might have play-by-play commentary for an intercepted pass, and color commentary for the situation in which a team has had four or more intercepted passes in the course of the game. A single interception deserves a certain kind of comment, while a string of them needs something different.
Here, as an example, is a list of just a few of the events we could detect in one edition of Madden. These are all from the section of the script devoted to running plays:
Long run -- resulting from many missed tackles
Long run -- not resulting from missed tackles
Run up the middle for short gain
Outside run for short gain
QB keeper -- resulting in gain
QB keeper -- resulting in loss (not a sack)
Goal line touchdown dive -- successful
Goal line touchdown dive -- stopped
I left it up to the programmers to decide what exactly constituted "many missed tackles." In some circumstances I wrote pseudo-code to explain exactly what should trigger a particular event.