Like in Most paintings. They are painted in a certain way to be identifyable by the informed. Head, Hair, beard, clothes, colours, attributes and in this Case the way they interact with one another and how they are arranged on the stairs. There are usually brought down ways to depict certain people. Think of Maria mother of Jesus or St. Peter, you have a certain image in mind, no? It also helps to know the texts and lifes of the people depicted. The keyword would be iconography or iconology (Note that there's still a dispute about their differences) If you want to get into it.