What characterises people who stay calm in crisis situations? When something critical and bad happens, like a house on fire with people inside it, people react very differently.


Some persons will usually panic, cry and can barely be soothed. 
Some persons will usually manage to stay calm, think rational and
actively do the best out of the situation.
Sometimes people go into shock, freeze
and are unable to act. 


For a long time I have wondered what is the difference between people with different reactions. What characterizes those who always seem to keep calm? What is the reason that some never seem to be able to stay calm? What happens mentally when someone freezes and neither panics/cries nor acts?

I suspect this has to do with Big5 Neuroticism or Big5 Openness facet #3("Feelings").
This is measured with the Big5 Neuroticism#6 facet: Vulnerability.
This trait measures how an individual reacts to stress, including emergency situations:


  High scorers on Vulnerability experience panic, confusion, and
  helplessness when under pressure or stress. Low scorers feel more
  poised, confident, and clear-thinking when stressed. - See more at:
  http://www.psychometric-success.com/personality-tests/personality-tests-big-5-aspects.htm#sthash.PmuKOUm0.dpuf


http://www.psychometric-success.com/personality-tests/personality-tests-big-5-aspects.htm

This facet has shown to be the one more important for being able to make decisions and depending on others (page 153):
\http://www.researchgate.net/profile/David_Watson14/publication/257044687_Procrastination_and_the_five-factor_model_a_facet_level_analysis/links/0a85e52fd0aa495d2f000000.pdf

This suggests that the panic, confusion and helplessness experienced may be due to being unable to make a decision on how to act and react.