Prosser, p. 104-105: Defense of Others
1. This privilege is similar to that of self-defense. Most cases involving defense of others relate to family members. There may be some question of what amount of force is reasonable under a given set of circumstances.
2. A point of disagreement is whether reasonable mistake applies to defense of others as it does to self-defense. Some courts say that the intervenor only gains the privilege in fact of the person he is defending, while other courts say that reasonable mistake does not negate the privilege.