Haunting characters

Ask anyone what they remember about their favourite book and it will be the characters that stick in the mind. We'll remember an outline of the plot, but it is the characters that will haunt our memories.  How can we ensure that our characters have this effect on our readers?  

 Focus in on unusual and memorable differences.  In Panthera : Death Spiral I wanted to turn the emotionless AI stereotype on its head, so I created Panthera, a poetry-writing sentient AI, who spends a lot of his time trying to understand humans.  He also has strong emotions.  One of my writing friends told me she really loved what I'd done with Pan's character, so I know the unusual combination of attributes works.  With Ren I wanted a professional woman whose whole world was devoted to animals.  Her brother Nic thinks that 'Ren's heart had always been animal.  Wild animal'.

A character's name can subvert our expectations.  Giving a low-down street character an aristocratic double-barrelled name immediately sets off questions in the reader's mind.  Is he a failed aristocrat?  Or a street rat with a jumped-up sense of his own importance.  Or perhaps he chose to turn his back on his title and wealth to work on the streets.  Three different reasons, which would result in three totally different stories.

We want our readers to live the story along with our characters, and that means showing their emotions.  Show us why your heroine is setting out on this crazy quest that will most likely get her killed.  And by showing how frightened of a dangerous baddie your other characters are you get across to the reader a sense of his power more effectively.

Uniqueness, details, and emotional responses are some of the things that make us remember a character long after we've finished a story.  Characters can haunt our consciousness for days after we've put the book down.  And if we can achieve that with our characters then we stand a chance of being remembered as authors too.