Motive in a mystery novel is obviously essential, we need to know why the killer was willing to risk everything to commit murder. The motive is usually the undoing of the murderer, once the protagonist detective figures out the motive, he can work out 'who done it'. But motive is not just for murderers. Motive is in every fiction character, or at least, it should be.
Motive drives every decision and every action for your characters. Why did the killer murder Old-man Hibbard? Stabbing him with scissors in the library no less? Did the man give the killer's new book a really bad review and suggested the pages be cut up for paper dolls or aeroplanes? There needs to be a motive, not just for the crime, but for the choice of tools and location. But what about non-murderous characters?
Why should our protagonist care enough to want to solve the crime? What is their motive? Perhaps they need to prove their own innocence, like in The Fugitive, or they need to protect the next possible victim, or maybe they're just addicted to adventure. There needs to be a strong enough reason for the protagonist to continue down a dangerous path, rather than just head home and turn on the telly.
This is true for all protagonists, no matter what type of journey they will be taking. Alice needed a motive to want to drink a suspicious liquid and shrink to fit through a tiny door, rather than just stay in the rabbit hole and wait to be rescued. Alice was curious. You can't have your character behave and then just explain the action later, you need to support the motive within the building of your character. We knew Alice was curious before she got to the door and the potentially poisonous drink. The author had built up our expectations so the only thing we would expect Alice to do is to move forward through the story.
Movement is the key to a good narrative, keep the characters moving forward through the narrative and you'll keep your reader moving through the book. For every character you should ask:
- Why are they there?
- What do they want most?
- What is stopping them?
- What are they prepared to do about it?
- Greed - character is usually seen following the money
- Power/Fame - this character is all about control
- Fear - possibly through paranoia or perhaps the threat is real
- Protector - someone the character cares about is at risk
- Revenge - driven by anger, bitterness
- Passion - love is the motivator and perhaps it's unrequited or there is a love triangle
- Blackmail - this character has a shady past they want to keep hidden
- Shame - this character acts through a sense of being unworthy
- Faith - even if it is misguided, it can be powerful enough to push a character
- A Rush - wild and untamed, this character comes across as spoilt.
Murder on the Orient Express - every passenger has a motive for murder!