I cannot stress enough, when writing someone, feel free to place your interactions upon their personality. Do not place your own ideas and goals. If the writer believes in anarchy, gun rights, and a pure free market, making your characters feel the same way makes them preachy. This applies to any belief. Writing can be a platform for your beliefs, but fiction probably isn't the place for it.
More, the character probably faces situations which we never encounter. Even in realistic fiction, they probably are doing things rare in every day life. Sitting in traffic and being annoyed might work for some authors (probably dark humor of some sort,) but does not play well as well as, say, fighting a demon.
If I, Ryan, were challenged by a hostile demon, I would collapse in prayer and perhaps run away. But if I, Julos, half-angel and champion of heaven, were challenged by said demon, I would fight, and perhaps win.
The angel expects to survive, so if I were that character, how would I behave? If I were a righteous (literally if I were an angel) badass, I might smart off to an engaged demon, I might question his parentage and his ability to hold his liquor. Or some such.
Though why his marriage promise matters, I'm not sure. (Engaged demon. Engaged. Like marriage. Nevermind.)
If the writer's logic and behavior can be framed from within the life of a character, making them live and behave in a "real" way is so much easier. Many writers tell me they can do dialogue or action, but most struggle with one or the other. Hopefully, having a full, real, image of the character means the other flows more easily.
If a person has ever been in theater or done improv, this should might be simple. Pretend your character is a role, only the lines are all adlib. Make it up on the spot, but as the character.
Pen and Paper games are a great way to learn how to improv a character.
I find making up a voice for a character helps. Give them a way of speaking, a specific vocabulary, and watch their personality grow. Make them some stereotype in reality, maybe the person is from the deep south, or jersey, or India. Since we're probably not representing people actually from India or the South, but a fictional person who behaves the way we imagine some group acts, it isn't insulting! Fiction! In a bottle! Brilliant!