From personal experience – you need to decide well in advance if you are going for personal defense or if you are going to set up building/community defense. That is because it takes time – lots of time – to get to know your neighbors, turn your neighbors into friends, and then turn your friends into those who are as concerned as you are about defense and survival. If everybody gets thrown into the fire with no advance warning the chaos outside might look like a picnic when compared to the chaos inside.
Nobody needs to be as committed to the cause as you are, but without some sense of togetherness the situation will probably hurt you before the Bad Guys can even get to you. All this “tactical” discussion is no good if you do not have a team that is interested in carrying out tactics.