Leonardo's method of working on the Last Supper was unprecedented. The Last Supper is not a fresco. Leonardo's intense concentration and hesitant manner of execution did not suit the commonly used medium for mural painting, in which the pigment had to be applied quickly before the plaster dried, precluding any changes during the course of execution. Instead of fresco, Leonardo devised his own technique for mural painting, a sort of tempera on stone. 

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. The pigment soon began to break loose from the base and a process of progressive decay set in. As early as 1517, it was noted to have begun to decay. 

More than wood panel used in frescoes, the brick wall of the Last Supper has been subject to changes in temperature, humidity, and moisture. These factors have created serious damage to Leonardo's painting and place serious doubt on his new technique. 

With regard to his use of perspective, Leonardo was successful in employing it to create an entension of the refectory and thus provides an illusionistic effect. There have been discoveries of how Leonardo made use of this artistic measure. A hole into which a nail had been driven has been found, located in the temple of Jesus. The location is the key spatial focus of Leonardo's painting of the Last Supper. He drove a nail into the wall and radiated string in various directions to help him see the perspective of the room he was painting. 

While the Last Supper is a typical subject chosen for the decoration of many refectories because of the Eucharistic theme of sacrifice, Leonardo chose to capture the moment in which Jesus announces to the apostles that he knows one of them will betray him. The apostles are captured in their sense of astonishment immediately after this announcement. His conception and pictoral treatment of the subject forges a new path. 

It is significant that Leonardo chose to ignore two widespread and long-established iconographical compositions. First, the arrangement of the disciples around a circular or square table had been tradition until that point. This was developed by Giotto from medieval models, and also used by Duccio and Sassetta in their paintings of the Last Supper. The necessity in this setup to depict some of the disciples somewhat thanklessly from behind was a contradiction to Leonardo's deisre for an expressive characterization of each of the twelve Apostles. A circular table would not provide adequate opportunity for exploiting the dramatic element of the scene. 

Secondly, the Last Supper's initial appearances as Christian iconography illustrated two main ideas handed down in the Gospel texts: reference to the betrayal of Jesus Christ and the counter-motif to the betrayal. These ideas were realized in prior portraits with the image of Jesus feeding his traitor, Judas, a piece of bread dipped in wine, and John reclining his head against the breast of the Lord. It was from this tradition, familiar to all predecessors, that Leonardo chose to depart. His conception of the theme was completely dominated by the idea of bringing out the announcement of the betrayl as the dramatic central motif. 

The faces in the painting, with the exception of Jesus (center figure), are reportedly those of actual people Leonardo sought out in Milan. Reportedly, Leonardo spend much time wandering through jails with Milanese criminals to locate the an appropriate Judas (fourth figure from left of painting). In addition to using living models from some of the disciples, Leonardo surrounded them with objects then in everday use. The tablecloth, knives, forks, glassware, and china were all similar to those of the monks residing at the monastery. 

Additionally in its form, the painting portrays expression through the agitated movements of the Apostles. Leonardo believed that painted figures ought to be represented in a way that those who see them will be able to easily recognize from their attitudes the thoughts of their minds. His Last Supper exemplifies that belief that figures should express emotional and psychological realism. The Apostles are arranged in four groups of three with Christ in the center. Leonardo's depiction of Christ as the focal point in perspective and in the form of a triangle, symbolic of the Trinity, provides for calmness and stability, whereas the gesticulation and facial expressions of the Apostles conveys their sense of astonishment. 

The dominant position of Christ is emphasized by the empty space around him. The background doorway frames his figure against the view of the countryside as his hands point silently to the bread and wine. His glance too follows this direction and places emphasis on the orderly arrangment of the objects on the table before him. To the left and right of him objects immediately fall into disarray. Thus Leonardo provides the space before the Lord as a symbol of the sacred action Jesus is ready to accomplish - offering himself as a sacrifice in the form of bread and wine. 

Leonardo kept Judas within the company of his fellow Apostles within his depiction. In earlier paintings of the Last Supper, Judas had been shown to the side of the table as he was fed the bread dipped in wine by Jesus Christ in an effort to display him as shunned. However, as the fourth figure on the left, Leonardo portrays Judas as recoiling from Jesus. He is the only figure whose face is lost in the shadow, a subtle indication that he is lost from the light of Christ. He is also the only individual other than Christ to not be portrayed in the wave of emotion that seems to increase from left to right in the painting in an attempt to symbolize his guilt.