The reason I'll be harder on Sims than Paralives.

@Hermitgirl if you have an idea for a Sim then give them the corresponding personality and they will act accordingly. It's how better personalities are supposed to work so sims don't feel like puppets and are also not completely out of control as well.
Unless you are going for really restrictive gameplay then I understand not wanting them to act out but you could always turn off free will for that.