Have you ever finished reading a book of fiction and were disappointed it ended? And was part of that disappointment because you were going to miss the characters? You actually liked the characters so much you didn’t want the story to end. You enjoyed your time with them. Why do you think you liked them so much?
The answer is very simple. The author did a fantastic job creating them. But how were they able to be that effective in their creation? It lies in the attention to detail. It goes beyond physical appearance. Go deep within your characters, their minds: how they think, feel, react to certain things, mannerisms. Study other people around you and make notes in a journal. What do you like and dislike about them? Make a list of habits and choose some for your character to have.
Also, consistency is a huge key to creating believable characters. Don’t have them be unafraid of spiders in one chapter and afraid of them in the next. Don’t give them one habit in the first part of the book and it not be their habit in another part of the book. That will only serve to frustrate the reader, and they’ll put the book/story down.
You won’t know if your readers will like your characters or not, but if you do your job and pay attention to detail, the chances of them having such an impact on the reader will go up.