When I began writing Bloodgifted, the first book in my Dantonville Legacy Trilogy, I knew that to make my characters believable, each had to have their own unique set of characteristics—their little personality quirks, ways of speaking and even history. Creating full-bodied entities requires research, and for that I used my background as an English teacher. In class I often set my students the task of creating profiles of the protagonists in the novels they were studying. It's a great exercise, not only in familiarising yourself with a character but, in giving them life.
Here's my character profile for my male protagonist, Alec Munro:
Profession: Medical doctor and researcher
Appearance: Straight black hair; lavender eyes (brown before his transformation); full mouth; serpent & sword tattoo on the left side of his breast. Height 6'1"
Family: Belongs to Luc's clan.
Sired by: Lucien Lebrettan (Lucius Antonius Pulcher).
History: Doctor and widower (wife and baby died in child-birth) who enlisted in World War I; stationed in a military hospital on the Western Front. Accidentally shot by Owen Dantonville and transformed by Luc in 1918; founder of the Munro Research Labs in Sydney, which specialise in treating rare blood disorders; appointed Princeps (leader) of the Brethren in 1960.
Character: Decisive and determined, he acts on observation and experimentation then uses whatever means necessary to achieve his ends; has a strong sense of justice; compassionate and protective toward humans; loyal to his sire, Luc, who he regards as a friend.
Personality: Dry sense of humour; enjoys teasing Laura.
Habits: Runs his hand through his hair when he's working out something; calls Laura "honey" when he's concerned over anything.
Best Friend: Jake
Favourite Expression: Damn! Holy mother of….
Car: Drives a blue Mercedes Sports.
I've done this profile with all my characters, occasionally adding extra details as they appear. I then print them and stick them to my noticeboard so they're at eye-level when I start writing. Now they're no longer in my head, but right there, looking down at me, willing me to bring them to life. And I do.
Try it and see how it improves your character development.