I think state laws differ slightly in what you can do. You should research this for your state.
Some basic ideas: take cover and be ready to defend yourself if threatened. If you have family with you, this should be your primary concern. If someone is nearby and about to be shot, consider taking action to protect that person.
Running around with your gun drawn is a very bad idea.
Here in Nevada the standard is you have to feel your life is in danger. That said no jury in this State has ever convicted someone of saving another person.