For example, in The Hunger Games Katniss's first and overriding motivation is save her sister from competing in a deadly tournament--Fear for her sister's life. However, the whole of a novel is not only filled with motivations and reactions, it is driven by them.
Motivation, action, and reaction lead to motivation, action and reaction these are the turning cogs (from the first to the last page) which drive any novel forward.
Throughout the book, Katniss must make decision after decision and for each one there must be a clear motivation. You can not just have Katniss decide to defy President Snow, you must show why. You must show her motivation.
These motivated actions lead to reactions--MRU (Motivation, Reaction Units)
MRU - Motivation, Reaction Units
The purpose of MRU is create logical and clear prose.
Motivation is the catalyst, the cause of the character to react. It must happen to protagonist or his environment.
1. Description: observation of events and surroundings, description indicates an external motivator.
2. Internal Monologue: own thoughts can be an internal motivator
3. Action: an external motivator to the protagonist
Something the character does in response to something else.
1. Internal Monologue: this will indicate an internal reaction
2. Emotion: involuntary physical reaction (Goosebumps etc.)
3. Action: if protagonist is acting it will be in reaction to something
4. Dialogue: if he is talking it will indicate a reaction to something
It is vital to get the correct order of the Reaction. Consider this from The Hunger Games:
...trying to remember how to breathe, unable to speak, totally stunned as the name bounces around the inside of my skull...There must have been a mistake..."Prim!"
The reaction half has three distinct responses which need to be presented in order. Firstly, you have feelings and thoughts (Involuntary subconscious response). This is the followed by action. This can be involuntary action like sweating or breathing hard (Involuntary physical response & Conscious physical response). Finally, you have speech. You do not need all three but they must happen in order.