I have spoken to and worked with many serving military personnel and veterans with PTSD and have heard some horrific stories that match some of my own. I never ask for any content of stories and just allow them to tell me what they need to say out loud, in their own way.
There are surprisingly innocuous triggers for flash backs, which is one of the many frustrating aspects of PTSD. Those who suffer with it find themselves getting lost in a flash back from minor little daily events that have no apparent connection to anything they have experienced. This recollection is from someone with PTSD and will hopefully go someway to help you understand what PTSD sufferers can be going through on a daily basis.