Spend time with them doing what?
Just working with them.
RP with them to better understand their mindset or characteristics.
Spend time thinking like the character and imagining what they would. Also imagine both the good and ugly part of them.
Maybe write in a way that sounds more like them than you when you do said RPs. Like, for example. I use the word ya, over you often when I talk. It's a speech quirk I picked up when I was young and even prefer to use the word ya when I write online as well over you.
But for one of my characters, I write in a way that sounds different from my speech. My OC Raffles used the word you and never ya while talking. His mindset was more cynical so I worded my sentences in a more harsh manner for Raffles than how I would word them normally.
Spend time thinking over ideas and figure out how to go about said ideas down the road.
It's important to treat a character like another person with their own ideals, beliefs, and personality characteristics. As well, as body movement if you want to go into super detail.
Just learn more about them as you would another person.