The ability to act swiftly and with determination is the single most defining sign of a man's character in The Man in the Iron Mask. This makes sense, seeing as this is an plot-heavy novel driven by action. Most importantly, a character's actions answer the question for us: is he honorable? Biscarrat is ashamed of causing the death of his companions, and so volunteers to be the next likely victim. He's honorable. Fouquet goes to rescue the real king even though it's in his interests to let the fake king rule France. He's honorable. D'Artagnan obeys the King's orders to arrest Fouquet even though he would really rather let the man go free. He's honorable. It is by their actions alone that you should judge the character of the men in this novel.
Each of the characters' occupations gives us an insight into their temperament. Being a king, for instance, carries with it an associated nobility and ease of command. By virtue of pretending to be the King, Philippe feels his consciousness expand into a certain kingliness. Being thrown into the Bastille of course brings about quite the opposite in Louis. As captain of the Musketeers, D'Artagnan's profession reflects the loyalty he feels to the crown of France.
Dumas will often explicitly tell us what we need to know about a character. This kind of direct characterization is helpful for analyzing the novel. For example: "Worthy Porthos! Born to help other men, always ready to sacrifice himself for the safety of the weak" (51.2). From that we get that Porthos is selfless and kind.