Imagine, if you will, plunging a knife into the organs of another living creature. Through the skin, through muscle and sinew and into a beating heart.
It's probably not something you've ever thought about or visualised in any real detail. It's almost certainly not something you've ever done.
When we think of murder, we rarely consider those cold mechanics. Yet that is the reality of taking a life, says Sir Kenneth Branagh, who is reinventing one of Agatha Christie's most beloved characters, Hercules Poirot, in Murder on the Orient Express.
"One of the things I did early on was get a knife - this was all under instruction from a safety expert - and plunge it into the organs of some animals. They were dead," he clarifies. "In order to go into the idea of steel in flesh being a really difficult thing to do. The idea of murder not being a game but involving a horrible mechanical act.
"If you're a stranger to it, the act of killing is a weird one. I kept trying to shake myself out of any sense this was a board game or drawing room mystery. Instead, this was about a dark dirty act that you need to know the reality of."
Unlike previous adaptations, which have played up Poirot's eccentricities with almost pantomime-effect, Branagh wanted to bring a darker, more serious edge to his version. Both he and the cast spent time studying the psychology of grief and its effects on people.
"What it's like to lose people and what grief does to people. It can turn them very powerfully in terms of an emotional reaction. All of that was to set up a danger and volatility on this train that was a million miles away from heritage, biscuit tin, pantomimic, drawing room mystery," he explains.
The Oscar-nominee leads the action both on screen and off, directing the film and its A-list cast, whilst starring as the famous Belgian detective.
"Directors and detectives, in as much as they have any connection, both seek truth. They look into the eyes of people they're interviewing and want to understand whether they can believe them," he says.
"The two things were bound together. Willem Dafoe said to me, he felt it was a very natural situation because Poirot runs the investigation and I end up running the film set... I think the two things were absolutely woven together. I've done this a bit but this seemed the most appropriate marriage of the two jobs."
Those jobs saw him working alongside one of the most star-studded casts in recent history - including Dame Judi Dench, Johnny Depp, Michelle Pfeiffer, Penelope Cruz, Willem Dafoe, Daisy Ridley and Olivia Colman.
"We literally started with Judi Dench who seemed to me great casting for Princess Dragomiroff, she's great casting for anything. I knew she would enjoy this imperious superior character because it's very far away from who she is and she likes to play surprises.
"Judi became a magnet for the other people. People like Johnny Depp and Michelle Pfeiffer were completely in love with Judi and when they came on board that was a domino effect," he explains.
"Ninety per cent of the success of a film like this is the casting because aside from the big landscape, which we try to exploit, the other forensic landscape in a 70mm version of a crime procedural is the human face. So you want people who can be there, hold the screen, hold your attention. Whom you are interested to watch think.
"We were reliant on finding people who could do that but also would be ready for the ensemble nature of things, who would enjoy the teamwork. This turned out to be a very larky group of people, who were very seriously focused when we were shooting but as soon as we said cut, were very very light hearted and badly behaved."
Of course, he won't reveal just what that bad behaviour entailed. Likewise, the cast are keeping mum on another set secret - the name of Poirot's outlandish moustache.
Almost a character in itself, Branagh's fulsome grey moustache is a sight to behold - a far cry from the delicate black twirl of David Suchet's Poirot.
So was it real?
"It began real but it took such a long time and it took up such a lot of my face that in order to do the maximum amount of grooming - and practically, this double swirled handlebar of such width needed so much grooming to protect it from the cold and damp - that it was better to be off me and on a bust of me and groomed so that we could replace it easily.
"There were quite a few of them as well, with various names. But I can't reveal the names. It took probably six months to work out what would the shape be and could I grow it. Eventually we came up with this double handlebar."
Modelled on a period photo of young soldier from the Belgian army, the moustache was just part of Branagh's transformation to Poirot. He also spent several months working with a dialect coach to perfect Poirot's particular inflection.
"I don't speak other languages so learning to speak in a French accent meant trying to learn to speak French to begin with, then how to speak French with a Walloonian accent, then with a Walloonian accent of a man born in Spa, who lives in London. All of that was quite specific. Otherwise you could be generic and sound like Inspector Clueso. He's not that. Everything about it had to have as much detail as possible."
Who: Sir Kenneth Branagh
What: Murder on the Orient Express
When: In cinemas next Thursday