I'm writing a book about a Human colony expedition that arrives on a planet and finds that it is populated by natives. Iron Age tech. The main character is the brother of the ships' captain (he has a boss, the owner of the expedition), and becomes an unofficial ambassador. After initial shock the humans hit it off well with the natives, they even sell the humans, but an incident triggers tension leading to the human leaders to start oppressing the natives. The captain is one of the ones who does this, and that is inevitable in the story. But for the first chapters of the book, already past 40k words, he is a very decent guy. That must change but I'm worried if it's too abrupt it will seem forced. Any tips on character changes of this sort would be appreciated.
Personally, doing this I would hint at these changes prior to the actual change. For instance, if you have a character who is nice who you want to become an extremely violent or angry character, show brief flashes of this throughout, or show hints leading up to this. Also, make sure that the change is actually brought on by something, whether it is internal or external. If the change suddenly occurs, without anything happening to cause it, it will seem inplausible.
You can also give the captain a strong reason to do what he does - good people can also do stupid things.
I think it helps to not think of character change as qualities/flaws added or removed from the person, but rather an alteration of the balance of already established traits. People don't suddenly acquire new traits, I would be so bold as to say no matter what happens to them -- but something dormant or suppressed can begin to dominate other traits. So, like Headintheclouds says, show us that this aspects exists in him to some minor degree before it takes over.
It can also be about revealing flaws which were there all along, but which only come out in the right (or wrong) situation.
For the expedition owner I already have put in hints at his character. The incident which triggers is that the natives believe the mountains to the north are sacred and refuse to allow the humans to mine there, even though the humans gave the native an extremely generous offer. After the final refusal the expedition leader decides to mine anyway, in secret. The captain goes along with the idea, and keeps it from nearly everyone, including his brother. It has been established already that the captain is very loyal to the expedition leader, and the ship is several years from Earth. It's when the mining is discovered and the demand it stop right away, that triggers disharmony between the humans and natives. And hence the captain's change. I've reached the point in my writing where the forbidden mining has already started but it hasn't been discovered yet. But it likely will be in a couple chapters. So is the captain's loyalty to the expedition leader, the long distance from home, the captain's feeling of responsibility for the colony which was running low on some rare metals, and the angry response from the native leaders when the mine is discovered enough to make the character's change reasonable? Also the brother, who's the MC, has grown close to the natives so he naturally has to take a stand as the natives begin to be treated harsher by the humans.
