In systems theory we have talked about how the pull and tug from individuals within the system actually keep the system in homeostasis. This means that when one person begins to fight the system another person will arise in the system to pull the person back into the system or to equalize the pull. This makes it difficult for systems to change.
In order for a system to move towards a true change, which would be a move in the homeostasis mid point, the forces acting within the system must be greater in one direction so that it overpowers the rest of the system. For example lets imagine a family who are at point A. Joe is in the family and he is constantly rebelling against the family norm which causes a lot of stress for the rest of the family. On the other hand we have Suzie who is a little angel who is balancing the system and alleviates stress for the family. The family has learned certain unspoken rules to deal with the stress but in truth are not very functional. One day John and Sue (parents) get tired of it and send Joe to counseling. Joe gets better and is no longer a pain, but when he gets home the unspoken rules are the same and Suzie begins to rebel causing homeostasis again. You see the system did not change only the different members roles within the system. In order to change the system the above family goes to counseling together and they all begin to see that the unspoken rules are dysfunctional and they all begin to change. This moves the system to a new state of homeostasis in which it is more functional. Suzie (the new rebel) still rebels, but because system rules have been modified, the system moves. Phew...I hope that all is as clear as mud :)
Now looking at our characters. Our MCs are generally our change agents. They are the ones pulling the system in one direction. However, they can't change the system until they change the system rules. They need to bring awareness to the majority of the members that there is a problem with the rules and that new rules xyz would be better. Then the system needs to agree with this.
A simple example of this is Charlotte's Web. Wilbur is born and lives at a farm where the norm is to eat pigs. Charlotte comes along and spins webs that change the norm so that they don't eat Wilbur. Charlotte is the change that moves the system and changes the system rules.