Roxanne Blouin-Payer shows in this GDC 2017 talk how Ubisoft created the emergent AI of Watch Dogs 2.
I personnally enjoyed a lot Watch Dogs 2, and I would say that it has in my opinion surpassed Grand Theft Auto V in its open world approach.
Even if Watch Dogs 2 was marketed as a "GTA-like", it has nothing to do with this genra. The character you embody in this game isn't a psycho-killer like Trevor or Michael from GTA V : he is the exact opposite, a nerd who both wants to help people in their daily lives and loves to do pranks with his hacking tools.
Often the player is encouraged to use his drones to resolve missions, without even entering the mission location with his character. The game has been designed to encourage the player to use non-lethal weapons during combat, so it could fit the main character's writing.
So to make the player care about NPCs lives and avoid killing them, Ubisoft had to create an AI that felt human as much as possible. In other term, unique.
People in the streets are singing, dancing, talking, fighting because a husband cheated his wife, smashing cars with baseball bats, being arrested
by the police, playing with dogs, dealing drugs, etc...but mainly, they react to your actions.
You can prank them, moke them, insult them, but also thank and sooth them...
Animation was one of the main vector to shows this to the player. They had to feel unique and responsive. They had to create lives that felt as important as the main character's one.
I hope Watch Dogs 3 will allow us to carry people whe just hit with our car to the hospital so we can try to save them. I hope we'll be able to follow drug dealers to their houses and see their routine so we can try to help them have a better life. I hope we'll be able to dig deep into NPCs lives, because their sharing with us the same world.
They are the ones who really suffer the consequences of our actions.