It’s not hard to find women with integrity. Most prefer to own self-descriptions that include terms like dutiful, responsible, calm, effective and efficient. But find one that is not stubborn, who can’t consider others’ emotional reactions before they act, and who refuses to blame themselves when they become overwhelmed and can’t deliver on promises? Good luck.
All characters in our stories must bear weaknesses if they are to be believable and endearing. When writing a scene that contains an ISTJ female, instead of focusing on their qualities, such as the strength of their will in creating or maintaining order, highlight their inability to be sensitive to the needs of others. Or maybe create drama by having this character not bend the rules, or become highly judgmental of others with opposing views.
Do you want to get deeper into their personality? Go to your quiet place and conjure a memory, the darker, the better, of a time when a mistake tore at your self-image. Dig deep. Open yourself to the pain of the moment and how you thought, at the time, that you would never, ever live it down.
Write from that dark place, and give your characters flaws.