What they could have done depends upon when they decided to take action. (1) Germany was not a totalitarian society when Hitler came to power...there was a window when people could resist. (2) Many people did resist in many ways all through the war years and many of them paid a terrible price and were murdered. However, many of them survived, lived to write books about the Nazis, the camps, the resistance movements they participated in. There is a vast literature on this subject. Books on resistance are not hard to find if you are interested.