It is the moment - the dramatic, irreversible moment that everything changes. The stunning, nauseating moment you realise that you might die; that in all probability you will die; that the humdrum, everyday things you were just doing are now part of another world, the normal world. It is as if you pass through a door. Will it close for ever behind you?
For Ben Innes and 61 other passengers and crew on board an Egypt Air flight 181 last week, this moment will have come shortly after their aircraft took off on an internal flight from Alexandria to Cairo. As they busied themselves with laptops, or settled back for a snooze, Seif Eldin Mustafa, an apparently unremarkable man in his late fifties, stood up and revealed that he was wearing a suicide belt. Suddenly, passengers were forced to confront a very different reality - one that involved them being blown from the sky, their lives coming to an end in the most violent of circumstances.
The instant of realisation that such traumatic life-and-death events are not happening to other people but to yourself is one of the most bizarre experiences we can undergo. Fortunately, most of us never have to. But, believe me, if, God forbid, it ever does happen to you, don't count on knowing how you will react.
That sheer unexpectedness is what is so wonderfully delightful about Ben Innes's decision to wander up the aisle and ask for a photo with the man who, as far as he knew, had Western passengers at the top of a kill list. Innes, like everyone else, has spent the last few years being bombarded by the grim images of Isil's very real torture and murder. He won't have forgotten, as Mustafa unveiled his explosive harness, that a Russian plane was blown up flying out of Egypt just six months ago. He couldn't have convinced himself, just days after the attacks in Brussels, that everything was going to be fine.
But when the chips were down, what was his response? To saunter up to the bomber, rope in a crew member to translate, strike up a conversation, get permission for a photo, and grin. Boy, that grin! "I'm not sure why I did it," he said later. "I just threw caution to the wind while trying to stay cheerful in the face of adversity. I figured if his bomb was real, I'd nothing to lose anyway."
Ben Innes, I salute you. I hail your impudence, your spirit and your determination not to take life too seriously - even as it appeared about to come to a close. Your friends say you are a chap who lovers "banter", and that your actions were entirely in character. But I know a little secret. I know that faced with the ultimate crisis no one has any idea how they will behave.
The reason I know is because of a cold, rainy 2003 day just before new year. I was with my driver and fixer on the outskirts of Basra, southern Iraq, after spending Christmas with the British troops based there, when a car accelerated around our nondescript Toyota and jammed on the brakes. Men with AK-47s climbed out. The first thing to note about such situations is one's utter naivete. Even as the men loomed menacingly towards us, I was wondering what they were up to. We are so used to being spectators to a drama that only reluctantly does the brain grasp when we become actors in them. I was still searching for some innocent explanation for what was happening when my door was wrenched open and a gun was pointed at my head.
It was then that I had the moment. It is quite hard to describe now. I do not write about the surreality of being quite sure that one is about to die to grandstand, believe me.
I couldn't understand what was being shouted. But I wasn't really listening anyway. To my shame, I took absolutely no notice of the fate of Saleh or Abu Salah, my brave and loyal colleagues. In fact I took very little notice of anything at all. Ushered to the side of the road, I simply assumed I was about to be shot.
Sure, I didn't ask anyone for a quick photo. But I did behave very bizarrely. I didn't look anyone in the eye. The whole thing passed as though I was distanced from it, as if underwater. And instead of stopping by the side of the road, I just kept walking certainly not trying to run, or escape. I was simply cocooned by the knowledge of what was to come, by its certainty, which made my actions altogether irrelevant. Weirdly, I felt supremely, utterly calm.
Col Tim Collins, who has faced many risks in his service with the Royal Irish Regiment and special forces, is all too familiar with that sense of calm. There have been two occasions when he felt certain he was about to die. "You accept it," he says. "A huge calm descends."
But what about those moments of crisis, when your fate is still in the balance? How do we behave then? By the time Innes posed for his now celebrated, if misnamed "selfie", the initial shock had passed. He was alive, if still in mortal peril. As he discovered, such situations can prompt reactions every bit as unexpected as the calm that can come in the face of certain death.
"Most people will respond in any way to keep themselves safe," says Dr Abigael San, a clinical psychologist used to dealing with the consequences of traumatic experiences. "You will have that fight or flight reaction, be more vigilant to signs of danger, and tend to interpret ambiguous signs as dangers. Your body is mobilised, everything is quicker, faster, stronger. Many people can react out of character, more aggressively perhaps."
She thinks that, as with Innes, when the threat becomes overwhelming, the instinct for self-preservation can give way to something else. "If you think you might be having your last five minutes on earth, you might want to get something done. Be remembered. Go out with a bang, as it were."
Certainly that is not what happens to everyone. Quite the reverse. And often it is precisely the people who you expect to be most gung-ho who are the ones to crumple in extremis. "In my experience, 80 per cent of people behave much as you would expect, and 10 per cent just crumble, which is piteous to see," says Collins. "On the other hand, 10 per cent step up, out of the blue, which is amazing."
Often, it is everyday men and women whose nature is not to seek glory who prove to be heroes. "There is something buried that is exposed for that moment," says Dr San. "It can actually make people behave in a way that can leave them feeling guilty afterwards. They remember coming across in a way that was a bit mad, a bit uncontrolled." I wonder if, now that Ben Innes has faced his aghast mother, and understood what he put her through, he might be feeling the odd twinge.
Such "madness" is not about bravery. Experience and commitment can teach us to be brave. Being brave is often key to survival. In my experience, we all carry around with us a reserve of courage which, when exhausted, it is near impossible to restore. Major traumas drain those reserves. Sometimes, however, they do not finish them, and it is left to comparatively minor incidents to tip people over the edge psychologically. "People can be traumatised by anything," says Dr San. "They can experience massive traumas and not have a massive reaction, see horrific wars in which their families get killed - then have a small accident that lifts the lid on their fear."
That seems about right. In the summer of 2003, I was caught up in a big suicide bombing in Najaf, central Iraq. At the time, despite the chaos and carnage, I was fine. Later, we had to return to the scene to cover the funerals of those killed. As we drove down, the veneer of professionalism cracked. In contrast to the calm I felt outside Basra, when I was far closer to peril, I felt sheer terror. I remember struggling, desperately, to pull myself together.
So one should never take for granted the ability to operate between the jaws of death, willingly and where others are incapacitated. In reality, it seems to spring from nowhere and in the oddest of circumstances.
"It's a mystery of human nature," says Collins. "Some people just shoot forward and do incredible things. Afterwards they often say: 'I only did what anyone would have done.'
"But that is simply a way of explaining it to themselves. Because it's obviously not true."