Drama is action. The word comes from the Greek word for "to do." Most all plays are a series of actions. As such, it's probably no surprise the characters in Saint Joan are defined by their actions. The Chaplain's viciousness is shown in his relentlessly maniacal pursuit of Joan's execution. Cauchon and the Inquisitor's compassion (at least to their way of thinking) is shown in the lengths to which they go to try and get Joan to repent. King Charles's lack of love for warfare (some call it cowardice) is shown when he decides to negotiate for Paris instead of supporting Joan in her conquest. And, of course, there's Joan, herself – a woman of action to say the least. She single handedly inspires a movement which unites all of France. Her actions define her as brash, daring, and maybe just a little bit proud.
The world of Saint Joan is strictly divided according to social status. All the characters have their specific place, which grants them certain rights, privileges, and powers. Look at Joan's trial, for example. Bishop Cauchon is the ranking official for most of it. So, he presides over the proceedings. When the Inquisitor shows up, Cauchon must defer to some of his power because the Inquisition carries a lot of weight. Below Cauchon and the Inquisitor you have the lower Church officials like D'Estivet, the Chaplain, and Courcelles. Though they would dearly love to, they don't have the power to excommunicate Joan on their own. The decision ultimately rests with Cauchon and the Inquisitor.
In the political world everything is just as regimented. There are kings like Charles and feudal lords like Warwick. Everybody knows their place. Then, of course, there's the army. We see everybody from commanders like Dunois down to the lowliest soldier. The conflict in the play is almost completely derived from the fact that Joan bucks all of these carefully laid out structures. She's just a common girl from the country. When she presumes to boss around kings, bishops, and generals the world is shaken. Her character is defined by the fact that she carves her own place in the social hierarchy through sheer resolve and determination.
Joan's choice to wear men's clothes is one of the big reasons she is executed. It's unheard of in this time. Everybody else wears clothes befitting their station. Dunois wears his armor, the Bishop wears his robes, and the ladies of Charles's court wear the fancy dresses that they are expected to wear. Joan however, chooses to do away with the simple dress of an ordinary country girl and don the armor and clothes of a soldier. To her mind, it just makes sense. She's out there fighting everyday. How is she going to do that in a dress? Anyway, if she dresses like a woman, the soldiers will tend to think of her that way. Not only will they not be as likely to listen to her, they may even try to take advantage of her. Her choice to wear men's clothes is indicative of both her common sense approach to life and her flagrant flouting of authority.