And people didn't sit on hard chairs to eat, it was considered extremely bad for the digestion. Instead the sat back in chairs more like recliners so that the food was able to digest gently, meaning that meals were very long, drawn out affairs, from 3 to 4 hours. There's no way that Jesus and his disciples would have been arranged anything like in the picture.
However this all had a purpose as, Leonardo was making a painting that he wanted viewers to understand clearly. If he tried to show his Renaissance audience a large group of people with a view of the back of heads around a table lounging, it would have been extremely hard to show all of their faces, with their reactions, and expressions. He adjusted the situation so that all of the faces were easily visible face on, next to each other. so the painting is more of a display like a stage in order to show all of the disciples reactions rather than an account of what happened.
Da Vinci has planned this picture meticulously, adding very subtle hints to the picture such as the hand holding the knife it does not belong to anyone as it is not at the right angle for anyone to holding the knife. taking into account that Da Vinci was a master of drawing the human form it would seem very unlikely that this is simply a case of careless placement ,I think rather that as it is placed in front of Judas(the traitor) and that the knife is held behind a back that Da Vinci is showing the back stabbing nature of Judas.
Da Vinci has also made extremely good use of a focal point in this picture too, using Jesus’ head as the focal point. This was probably done to help emphasize this important period of Jesus’ life and to show that all attention was on him. If you follow lines that have been placed specifically to lead the eye to Jesus’ head then you can see that this is true.
as you can see I have drawn red lines showing the way in which the focal point has been created. These include beams, wall designs, the level of heads and Jesus’ arms. However if you look at the picture without the red lines it is very discreet and subtle. His red clothing also helps him to stand out above all others in the picture.
The picture could either be a documentation of the disciples reactions towards Jesus announcing there is a traitor among them or Judas revealing he was the traitor but since Da Vinci has used these techniques to draw attention to Jesus I probably think it was the announcement that there was a traitor present and all twelve have reacted to the news in different degrees of horror, anger and shock.
Another main purpose of the picture is to mark the introduction of the Eucharist, where the disciples are given precise instructions on how to eat and drink in the future, in remembrance of him. This is shown by the opened arm gesture pointing towards the food in anticipation of his death.