Readers forget plots; they never forget people. Ask anyone about their favourite novel and they will describe a person, not a storyline. After six books, my whole method fits on an index card: give every character a secret, a contradiction, and something to lose.
The secret is what they never say aloud — the letter unsent, the debt unpaid, the kindness nobody witnessed. It does not need to be revealed; it only needs to shape every choice they make on the page. Contradiction is what makes them human: the brave firefighter afraid of the dark, the gossip who keeps every confidence that matters.
Something to lose raises every stake
A character with nothing to lose can wander through any plot untouched. Give them a shop mortgaged to the roof, a daughter's recital, a reputation built over thirty careful years — suddenly every scene has weight, because the reader does the maths of loss alongside them.
Give them a secret, a contradiction, and something to lose — then let the story take it.
I keep a ledger for each major character: three columns, filled in before chapter one. Minor characters get one line each. When a scene feels flat, the answer is almost always in the ledger — I have forgotten what this person fears losing.
Let them surprise you
The index card is a starting point, not a cage. Twice now a side character has refused to leave the page and walked into a leading role. When that happens, follow them. Your outline will forgive you; your readers will thank you.
