Here is an etiquette question. Should I ask permission before I sketch someone? With photography, the answer is more obvious, I think. If I ask permission, and the person agrees, he becomes, for that short time, a model. He may be uncomfortable, wooden. Professional models are used to the intense focus of the artist's eyes. Other people find it unnerving. Staring at someone is extremely confrontational. Even a loving gaze will become strange if it lingers too long in one spot.
Still, some artists do ask. I don't, usually.
Public executions being a festive affair, da Vinci attended the hanging of murderer Bernardo di Bandino Baroncelli, who had been part of the conspiracy to overthrow the Medici family's rule of Florence. I doubt Leonardo asked Bernardo's permission to do the sketch.
In his notes, da Vinci described the clothing: "A tan colored skull-cap, a doublet of black serge, a black jerkin, lined and the collar covered with a black and red stippled velvet. A blue coat lined with fur of fox's breasts. Black hose. Bernardo di Bandino Baroncelli." If your kid plays the video game Assassin's Creek II, ask him about Bernardo.
Another famous field sketcher was sculptor Henry Moore, whose "Shelter" series of drawings documents the experience of hiding in the "Tube" (the subway) during bombing of the London in World War II. Moore is best known for his huge sculptures. See a fine example of one of his sketches below: