Now I feel like the magician who doesn't want to reveal his magic tricks.
Excluding the mannerisms system I want to include, the system would work on a list and probabilistics. The list will have behaviours from the most open to the most autistic (Im using the "Openess/Intimacy" parameter here as an example, but actually all five would be used at the same time). Then, with a random chance with the shape of a normal distribution (Gaussian bell) centered on the most common (or closest) response for X Openness Tendancy (cow1.opennessd in the game, betaguys) a behaviour is chosen. That behaviour will be an " empty sentance" that will be randomly filled with words and expressions depending on the personality. A lazy character would use short expressions and vocabulary for the same thing a energetic character will go overboard with explanations and interjections.
That's the ground principle. With it, the more text and behaviours learned by the system, the more unique each cow becomes.