This can be one of the most difficult to pull off, and even sometimes I can barely pull it off. Making three dimensional characters is a must in almost any book, if you ask me. But it is also the headache for many writers, even me to an extent.
The first thing which I would say would help make your characters three-dimensional is giving them a flaw. Or a number of them. Some small, some big. And enough to make them believable to a reader. Having one flaw can be quite boring, but a few where they would need to rely on others is more suitable instead. And one might just lead to their downfall.
Another is develop details, sometimes certain quirks or details help to make a character have a little more depth. Even more so when why did they eventually develop such a habit, it can be quite fun to invent the various back stories constructed in such a way to make them so.
Their quirks and habits, must reflect their mindset. Or at least give a nudge that there is a lot more than what meets the eye, or even any disabilities they might have. Not shove it into the reader’s face or simply mention it but also show it in some ways that they are as such.
And most importantly, is to have another side to them. Rather than the usual characters personality, show a little more to them. It can make the book a little funnier, a little darker, a little more deep. Also, it must be a smooth transition or hinted from early on that they might have such a side or personality. Only that it is hidden from them.
Most importantly, is to always be honest with the character you have and their motivations, their goals and their morality. Show what their mind is thinking, show what kind of actions they would take. And also, let their actions and words define them, you can justify it from their point of view but they should never change or become a gentler version of what they should have been.
It is also to portray them in their true colours if they are main characters. Otherwise, they wouldn’t even sound like themselves anymore, just hypocrites if done unintentionally can be rather off putting.
So, do you have any other ways to make a character three-dimensional? Feel free to leave it below, it is always welcome.