Giotto, The Arena Chapel Frescoes: The Last Supper
Scrovegni Chapel, Padua, Italy
John reclines, as usual, on the shoulder of Jesus. Peter, with the square beard, is placed at Jesus' right. This is also a common practice.
Jesus and Judas dip their hands in the dish at the same moment, a sign that he is the one who will betray Jesus (Matthew 26:23). As Judas does this, his halo starts to fade.
More of the Last Supper
Source: Website of Christian Ganne