Fresco vs Tempera
Dan Brown's in The Da Vinci Code calls the Last Supper a FRESCO.

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.

Normally, the way a painting like this would have been done would be a that the plaster is made wet, and then the painting is done on the wet plaster. When the plaster dries, the paint is sort of intermingled with the plaster. The wall was first coated with a strong base of some material which would not only absorb the tempera emulsion but also protect it against moisture. His base was compounded out of gesso, pitch, and mastic, and has not proved durable.

However, that type of painting is limited in its colours and can't be retouched. When the plaster dries, you're stuck.