Help! How do I distinguish between -은/는 and -이/가 when describing personality?

I am a grammar-focused learner trying to reach an intermediate level, but I keep getting tripped up on subject vs. topic particles. I understand the textbook definitions, but I struggle to decide which to use when introducing someone's traits in Korean. Does anyone have a mental framework for deciding which particle feels more natural when you're specifically emphasizing a fact rather than just tagging a subject?