It occurred to me recently that I have a preoccupation with deceit in my stories. This probably stems from my own trust issues but, certainly, the two full-length drafts I've completed have deceit and infidelity at the heart of them. So I began to think about the characters I've created and how they 'explain' their lies.
It's difficult for a start because none of the liars and adulterers are viewpoint characters. I tend to work from a singular first-person POV which leads to a very narrow perspective on the world. It's down to my protagonist to interpret the motives of the other characters and that can lead to misunderstandings galore. On the other hand, this narrow viewpoint allows the deceit to be continued - they can only comprehend what they see and what they are told about fellow characters.
In both stories my main character is the lover of a married woman. However, I know the motives of the women are quite different. The problem is conveying this to the reader.
Jude, for instance, is disillusioned with her marriage but content until she finds herself falling for someone else. The relationship between her and her husband is portrayed, but only through the secondary device of conversation about them and through the eyes of the protagonist. As Jude's motives in the story fall under suspicion, so the reader begins to doubt her feelings towards both her lover and her husband. By the end of the story I don't think I've redeemed her enough.
I like her. All the things beneath the surface that I'm aware of aren't coming through because of the viewpoint difficulty. Due to her history, when she says something the reader is unsure how to interpret her motives. This needs thinking out certainly. Much of the resolution comes from conversation but I think the old adage about seeing not telling needs to come into play. Perhaps the only way you can demonstrate love between two estranged characters is through action of some sort?
Marie, my other cheating wife, has motives which are much more complicated and don't just resolve around her emotions. As a result she is much easier to portray. There is no question towards the end of the novel whether she should be trusted as the reader has learned from her actions throughout. She feels less bland and more real than Jude does because I've let her show her true colours instead of insisting upon them in speech after speech.
In creating a cheater you have to clearly define their motives, whether it's love, lust, financial gain, manipulation or whatever. Very few people fall into affairs because they've got nothing better to do (though it's not impossible). If you can identify precisely why your character is risking their marriage (or even if they are at all, some marriages are more open than others) then you might be able to portray them more effectively.
As a secondary character in a first-person narrative it can be extremely difficult to show not tell. Alongside using the explicit telling and showing method don't forget to use foreshadowing, subtext, juxtaposition and various other techniques to influence how your reader is supposed to interpret the character.
In my own work it's clearly evident that while Jude is only on her second draft, Marie is on her fourth. That kind of character depth and understanding unfortunately only comes via revision for me.