One of the most common twists is the ‘Betrayal’ and I wrote about this once before. This is where a lot of depth and growth can come from. A traitor can ruin plans and destroy group cohesion in one simple act. They can also become a catalyst to bring characters closer together and switch to a new main plot. It’s definitely a tried and true twist with a lot of oomph. Yet it can also go horribly wrong.
Take Lando Calrissian who is the first ‘traitor’ that I remember running into during my days of books, movies, and TV. He was charming, smart, and quickly established that he was a businessman. There was also a slight edge to his initial meeting with Han and the others that I noticed on later viewings. So there was a feeling that he had something hidden, so the turn was a stunner to me. So was the second turn. Yet, Lando never appeared to be acting out of character. He did what he had to do for his city and then he turned again when the deal was changed. This might not be a perfect example because I’m still a little lost on why he was trusted so quickly after that, but that’s more on the other characters’ heads. Maybe Lando was just that smooth.
You might note what the key point is here: A traitor needs to have a reason. Either they were faking loyalty the whole time, they have a loved one at stake, or any number of reasons they do what they do. Most readers won’t accept that a character goes villain simply because he felt like it. Change of morals after a trauma or something to create the alteration of character.
Foreshadowing is a key component to a betrayal because it prevents it from coming off as a last minute decision. Even if that is how it originated, you can come back to put clues that it will occur. If you don’t want to hint at a betrayal then you can note a reason the character might side with the villains. A character doesn’t even have to go with the villains if it suits their nature and circumstances. For example, a beloved sister ends up getting killed in the crossfire of a battle. The brother is one of the heroes and he leaves them at a critical moment while declaring he will destroy both sides. This creates a third faction and can make things more interesting.
As with the previous posts, it all comes down to staying within the character’s personality and moral code. Would they turn on their friends if they’re beliefs are put into question or will they change their ethical path? That’s another option. Betrayal of one’s self would fall under this and can lead to amazing character growth. As usual, there should be some note that it’s possible like something he or she would place above their own morals. Some readers might not accept it because everyone has a different priority list, but you have to stay true to the character.