My method is pretty long winded, but I do it two ways. As I mentioned in another thread, one is to daydream as my ocs constantly. I think of random scenarios, or maybe common scenarios, and imagine how my character would react in that situation. This could be a scene i see in someone else's movie (like an impromptu kiss) it could be something I learn about in a video or an article (like the struggles of life in a wheelchair AND caring for children alone as a father).
Spending all this time imagining the character mindsets help me to develope consistencies for them, and even inconsistencies that make them more complex (why would a character who borders on a germaphobe use his shirt to clean something? because he cares more about getting the sand off his toddler's face than the mess)
The other way I go about it is by making a character design sheet. I find that the more questions I answer for each character, the more about their personalities I discover, and I can go back and edit/ answer other questions to make the character more consistent and complex step by step. Some of these questions are random (like how do they feel about pineapple on pizza and mint chocolate chip ice cream) but it helps me know where the characters stand in little nuanced ways. Others are more specific, like what is their favorite song. Oftentimes a "what" question will lead into a "why" I have to find an answer to, and that prompts me to build backstory, and sometimes inspires the backstory.