First and foremost, you need to learn, if you don’t know how, to observe. Look at people around you, listen to them talk, watch them walk, the hand gestures, facial expressions, the sounds they make when happy, distressed, pleased, upset. A lot of authors keep notebooks. I did for a while. Anything unusual I recorded.
Not doing much of that now. After nearly thirty years of teaching and watching (teachers are good watchers – or they should be) you learn to pick up on things. So the first thing is observing. If you want to make your character come alive, then you have to observe how people react.
However, an aside. A character is not necessarily a person. It can be a house, a tree, a rock, even a fish. You can probably name any number of characters that would never be classified as human, and yet, to make them come alive, we have to give them human characteristics. So first things first. Observation!
Tomorrow, recording those observations.