Steinbeck develops the characters through their dialogue. For example, the conversations between Lennie and George reveal a lot about their relationship and individual personalities. George's exasperated yet protective tone towards Lennie shows his role as the caretaker.
He develops characters by placing them in specific situations. When Lennie is in trouble, we see different sides of George. George has to decide how to protect Lennie. And for the other characters like Curley's wife, her loneliness is shown by her flirting with the ranch hands. Her actions are a result of her being isolated and unfulfilled. Steinbeck also uses physical descriptions. Lennie's large size and clumsy movements help to show his simple - mindedness.
Actually, 'Of Mice and Men' is not a novela. A novela typically has different characteristics and length compared to this work. 'Of Mice and Men' is considered a novella due to its concise structure and focused plot.