Authors develop a character by giving them flaws, fears, dreams, hopes and goals and by having them overcome or fulfill these things throughout the course of the story.
For instance, Neville Longbottom from Harry Potter begins as a shy, awkward boy that we know very little about. We know that his family comes from magic, yet somehow he is nearly useless with a wand. As the books progress, we learn more about Neville, his life, we see him go through experiences where he is forced to overcome his fears to protect himself and the ones he loves and in the end we see how these experiences changed him and turned him into a courageous young man who anyone would be lucky to call a friend.
If you are having trouble figuring out how to make your character's choices realistic, I recommend writing a character profile. I do this for every one of my main characters. Begin by answering basic questions such as your character's full name, nationality, age, physical description and then move on to his hobbies, talents, likes, dislikes. Then his relationships with friends or family (I try to do this for all of the characters he interacts with the most) and finally answer questions such as his biggest fear, the moment in his life when he was the happiest, any traumatic events he experienced, his goals and dreams.
While you will see that your characters really begin to come to life once you actually begin writing, these profiles are nice to have on hand so that you can go back and analyze how he would react to certain situations.
For example, If I wrote that my character was of South Korean background, had critical parents, had a fear of public speaking and once forgot his lines during a school play, it can be assumed that he would be horrible in social situations, probably not very talkative, probably doesn't have very many friends but is likely a perfectionist as he always tried to impress his parents. This could help you in developing a character in that at the beginning of the novel he is very closed off, quiet but then he meets someone to whom he opens up and you find out he is actually quite intelligent and perhaps this person who he comes to care for gets caught up in a bad situation and he is forced to go undercover to get her home safetly, forcing him to be brave in a social situation.
I hope this made sense. I rushed through this explanation.