Focus on this artwork and discuss the Stages of painting.
The first version revealed by the x-rays is in the Mannerist style of the most admired artist in Rome at the time, Giuseppe Cesari, with a crowd of small figures amidst massive architecture. It must have seemed static and distanced. The second version turned to Raphael for a model, adding a crowd of onlookers displaying fear and pity, including a woman who presumably represented the nun. This was in line with the crowded scene requested by Cardinal Contarelli and with the tenets of Mannerism, which demanded bodies and buildings defined by perspective and drawing, but Caravaggio had already developed a personal style in which bodies were defined by light and darkness and in which backgrounds were eliminated. 
At this point Caravaggio left off the Martydom and turned his attention to the companion piece, the Calling. This drew on his own earlier genre-pieces, Cardsharps and The Fortune Teller, but writ large. Apparently re-inspired, or perhaps with renewed self-confidence, Caravaggio turned back to the Martydom, but this time working in his own idiom. The third version dropped the architecture, reduced the number of actors, and moved the action closer to the viewer; more than this, it introduced the dramatic chiaroscuro which picks out the most important elements of the subject, in much the same way a spotlight picks out the action on a stage, but centuries before spotlights were imagined, and chose to represent the moment of greatest drama, as the murderer has plunged his sword into the fallen saint.  This is the version we see today, the action caught at the moment of highest drama, the bystanders reduced to supporting roles by the sharply selective light, the whole giving the impression of a moment seen as if in a lightning flash.
Cardsharps Raphael M Rome Caravaggio Mannerist Giuseppe Cesari chiaroscuro The Fortune Teller Mannerism