Successful leaders possess many traits, such as being consistent, positive, responsible and etc. Consistent as in being a tyrant today and the next day being very kind and thus confusing group members/employees. Positive as in encourage your group members/employees when they're not doing well and encourage them to do better even if they have done well. Responsible as in if your group members/employees make a mistake, it is the leader's responsibility to help him/her improve.
I think a leader should stay calm and think carefully before doing anything in a crisis situation as a wrong move could worsen the situation. He or she should also inform the group members/employees about what has happened so they do not get a shock when something goes wrong.