Revisit texts you have read and make a list of fictional villains that you find compelling and believable. You may also choose to include villains from film, television or history.
Analyze your list of villains to find commonalities and to zero in on specific traits that you would like your villain to have. Be sure to include both physical and nonphysical traits. Adding physical details, mannerisms and idiosyncrasies that enhance the interior emotional and mental processes of your villain will add to the believability of the character. Outward physical traits are often reflective of a character's inner state. For example, if you want to convey the inner ugliness of a villain, try giving that character physical deformities such as scars or warts.
Decide whether your villain will be the main character or the secondary character. The main character is the one whose point of view will be given to the reader.
Brainstorm the main conflict in which your villain and your hero will be engaged. List the ideas that your villain must espouse in order to be in conflict with your hero and the actions he will take in that conflict. Flesh out your villain's motives. Ask yourself why the villain thinks, feels and acts the way he does. Fleshing out your villain's motives may also involve developing his back story. The back story can detail events that occurred preceding the opening events of your text and impact the way a character behaves. For example, your villain's motive for being in conflict with the hero might be that the villain's father and the hero's father were also enemies decades earlier. You can refer to this back story at various points in your text in order to allow the reader glimpses into the mind of your villain.
Give your villain a distinctive voice within the dialogue that sets her apart from other characters. This may be achieved by incorporating one or more of the traits you identified from your initial list of inspirational villains. Perfect speech is not realistic, so try adding a speech impediment, extreme articulateness or wordiness to your villain's dialogue. This will make your villain's voice distinctive and can reveal additional information about her intelligence and physical state.
Allow the reader to see into the mind of your villain at times, even if your villain is not the main character of your story. Psychological depth will add to the believability of your villain. This may involve incorporating bits of your villain's back story into the narrative. Try having your villain recall a memory or make a revealing comment to another character. Alternatively, you may choose to use flashback to show your reader a scene from your villain's back story that may reveal his motives to the reader.