on Writers Helping Writers:

Trauma is defined as anything that’s overwhelming or unpleasant that causes long-term mental or emotional problems. Trauma rewires the brain and causes disordered thinking. So, if you’re looking for a way to SHOW a character’s trauma background, to show the WHY behind poor choices, irrational behaviour, etc., use internal dialogue that reflects this disordered thinking. This is the key to creating emotional connections for readers.

Below are some common ways that trauma causes problematic thinking patterns. Showing this flawed thinking, the emotional reactions to it, and the behaviours it causes will reveal to readers what’s important to your character, what inner obstacles they face, and often a whole lot about their priorities, values, and self-worth.