Q: How can I create realistic fictional characters?
A: It is important to remember that you are not a flat or one dimensional person. You have a multitude of feelings, and each one is like the facet of a diamond. Just like you do not have one emotion all the time, your character should not have one emotion all the time. During the highs and lows of the plot, your character's emotions should reflect that.
It is important to create a life history with your characters. Who are they? What have they lived through? Where did they come from? What are their dreams and aspirations? These are things you should have an idea of. You should know everything about their physical appearance. The way their eyes glitter when they smile... The way they crinkle their nose when they laugh... These are things you should keep in mind when describing the character within the story.
You should also have a good grasp of the physical or emotional struggles of the character. Think of your life. All of your relationships aren't always pleasant. Use that in creating your character. If you can use one of your relationships as an example, then it would make the plight of the character more realistic.
There is a lot of talk in reviews about showing, not telling, a character's emotions. That means instead of saying the character is scared, you should depict what they are physically going through. The accelerated heart rate... The blood pumping through their veins... The sweat dripping from their brow... Showing is the best way of capturing your audience and pulling them into the story. They will feel they are apart of the story which is exactly what you want as a writer.
Aspects that give your character a sense of reality is capturing their habits or quirks. For example: In Weeping Willow, when Willow gets frustrated, she rakes her hands through her silky hair. You may have a few habits or quirks that can be incorporated into your character. A habit can be seeking a quiet place when things get tough or slamming doors. A quirk can be pursing one's lips. It is just something a character does without realizing it which makes it more believable.