What strategies does Paton use to describe John Kumalo in Cry, the Beloved Country?

Paton's strategies to characterize many of the main characters in this book do not differ from standard author strategies. Paton uses a combination of direct and indirect characterization to build the morally corrupt person that is John.
Direct characterization occurs when the narrator directly tells readers about a character. Nothing is left to interpretation. Readers can see this type of characterization in chapter two, when we are told that John was a carpenter. Chapter seven also has a good example: readers are told that John is fat. Nothing is left open to interpretation.
This differs from indirect characterization, which requires readers to infer something about a character based on how that character acts and speaks and/or how other characters respond. The book tries to portray John as morally ambiguous or even amoral. Being told that John abandoned his first wife and is living with another woman outside of marriage tells readers that John isn't exactly a faithful and committed type of person. Readers can also figure out that John is quite self-absorbed or even narcissistic. He cares about himself and his own image. He doesn't believe in a cause other than garnering further recognition for himself. Readers aren't expressly told this; however, the following quote definitely helps convey that characterization.

There are some men who long for martyrdom, there are those who know that to go to prison would bring greatness to them, these are those who would go to prison not caring if it brought greatness or not. But John Kumalo is not one of them. There is no applause in prison.

In Cry the Beloved Country, John Kumalo and his brother Stephen Kumalo have very different personalities. John has moved to Johannesburg and largely lost touch with his brother. When Stephen comes to the city to look for his son, he goes to church and mentions his brother to the minister. In the early pages of the novel, Alan Paton primarily uses indirect characterization for John. This includes Stephen's conversation with the minister, from whom he learns that John is a well-known politician.
Paton also uses dialogue. After John and Stephen meet, they catch up on various doings since they last saw each other. This provides the reader with information but also creates an impression of John, both about the facts of his life and the way he wants to influence his brother's opinion. John prefers to speak in English, he talks about the oppressive life back home, and he indicates that he is not legally married to his current partner. In addition, Paton describes John directly, interspersed with the dialogue; for example, he describes John's smile as "cunning," indicating his is an insincere person.