Leonardo da Vinci was already a well known artist when he created his masterpiece The Last Supper. He painted it on the back wall of the dining hall at the Dominican convent of Sta Maria delle Grazie in Italy. The reason the painting is laid out the way it is is that Leonardo was trying to "extend the room", to make it look like Jesus and his apostles were sitting at the end of the dining hall. This painting became an instant famous work, for many reasons.
      Dan Brown's error in The Da Vinci Code is that he calls the Last Supper a FRESCO. This painting is not a fresco. This is a critical mistake because if the painting had been a fresco, we would still have a superb image to view and examine. It is *because* it is not a fresco that we have all these questions now about what the painting is showing us. In fact, a main reason why the Last Supper painting is so famous is because it is not a fresco - and therefore that it was in essence destroyed immediately after being painted. It's important therefore to understand this issue.
      Part of why Leonardo's version was so famous immediately was that he had chosen a very unusual way to portray the scene. Pretty much every painting of the Last Supper up until now had involved Jesus blessing the bread and wine - a nice, serene scene. Judas was always shown off in a corner, sulking, away from the rest of the disciples. This is what the Dominicans thought they were getting. When they instead got this "violent" version with the disciples all screaming and yelling, the monks were not exactly pleased. It created a lot of publicity.
      If you're curious, the top version of the Last Supper shown on this page was the pre-restoration version