The majority of the rules in the core rulebook operate under the assumption that all the player characters are on the same side and readily cooperate towards a common goal. The only people to potentially react differently are those who are encountered randomly, either in the form of Survivors or other members of The Living... this is far from reality, unfortunately.
Group dynamics are an important aspect of survival and a group that works well together will stand a much greater chance at succeeding. This may not be a factor that has to be introduced artificially by a GM. The conflict within the players themselves are almost a guarantee if you are playing a campaign. The players will have a very clear idea as to what strategy would work best for survival and doubtless there will be those that agree as often as there will be those that don't. How this conflict is to be resolved is an important part of running the game.