The main characters could be a married person and someone they meet unexpectedly. For instance, a married woman who meets a charming stranger and they start an affair. There might also be secondary characters like the spouse of the married character, who may be oblivious to the affair at first. Or there could be a best friend of one of the characters who starts to suspect something is going on and adds to the tension in the story.
Since I haven't read this particular 'the secret affair novel', it's hard to say exactly. However, in a typical secret affair - themed novel, there would likely be the two lovers at the center. One might be a character with a lot to lose, like a public figure, and the other could be an ordinary person who gets drawn into this complex relationship. There could also be characters who are enablers or those who try to expose the affair, which would create more drama in the story.
I don't know since I haven't read the novel. But usually, in a novel about a secret affair, there would be the two people involved in the affair. They could be a man and a woman from different backgrounds, like one from a wealthy family and the other from a working - class family.