Something really subtle, you probably don’t notice it often - but the AI in Halo 3 react in specific ways that are clearly scripted and not random, depending on the actions you take. Currently, I can think of a few examples, and would welcome people to point out any more that I missed.
Mission: The Ark
Probably the best example of this, the very last group of enemies in the level (the Brutes performing the ritual) will not attack you until you kill the Chieftain - as has been pointed out, it’s a challenge issued by the Chieftain.
Mission: The Covenant
In this mission, there are three instances of this. First, when fighting the Brute Chieftain in the first tower, ever notice how he won’t charge at, or otherwise attempt to attack you, at all - until you kill all the other Brutes? It’s a subtle thing that does make the AI feel a little more alive. Secondly, when entering the second tower and getting attacked by Drones, ever notice how if you enter one of the small hallways along the side, all of the drones will immediately fly into that hallway? Sure - this example seems like an obvious AI-thing, move to where you can see the AI, but go and experiment - they do it every single time. It definitely feels scripted.
The third is probably the best. At the beginning of the dual-scarabs fight, when the two hornets land, one of the pilots gets out. The other doesn’t. If you hop onto the side of the hornet with the pilot inside, that pilot WILL fly over to one of the scarabs, hover above it until you get out, and then pull back. Once you destroy the scarab’s core, that pilot’s hornet will land on top of the scarab to pick you back up and carry you to the next one. Granted, this one’s a bit “fragile” in the sense that it doesn’t work perfectly all the time, but when it does, it’s incredibly cool.
There are probably many more examples, but anyway - my whole point for bringing this up, is this is something I definitely hope to see return in Halo: infinite. Your thoughts?