There are three things to consider when writing as the mouthpiece of your character:
- What they observe
- How they filter it
- How they react
Only write what your character perceives.* Only write about it as much as they think about it. No one notices everything.
Do they Jason Bourne the room and count every door and suspect? Do they zero in on their love interest and notice nothing else? How likely are they to notice and internalize the hair and eye color of other characters? Do they care about clothing? Architecture?
The depth that your character internalizes what they see will define both their emotional interests, and their intellectual capacity.
Hint: Jason Bourne may count every weapon in the room at first glace, but he's not about to notice whether or not his enemy has her hair in a "slightly messy french twist."
How do they filter what they observe?
How your character feels about their observations will come out in the vocabulary they use to describe them. "He woke up every morning and ate Chocolate-marshmallow-frosty puffs" vs "He drudged out of bed each day to consume the recommended dosage of hydrogenated cardboard cat-piss." Same thing. Different filter.
Hint: Katniss may observe that Gale is good-looking, but her lack of extreme detail or adoring adjectives gives her a confident "need to man to complete me" edge.
How do they react to what they filter?
While you may not think of action as voice, it speaks volumes to your reader. As we all know, "action speaks louder..." So having your character follow through with what they observe, and how they filter it, will reinforce their view of the world. Why is this even worth saying? Have you ever read a book where what the main character thought, and what the main character did, did not match up? Mmhmm.
Hint: Harry observes the sorting hat ritual his first year, and we understand he's nervous because of the descriptive filters, but it's not until he whispers "not Slytherin, not Slytherin" that his truest viewpoint, and character, is defined.
*the only exception to this is 3rd person omniscient, but it still absolutely applies to 3rd person limited.
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