There are a bunch of little cues authors drop when a female character is possessed, and I always find them fascinating because they mix physical, behavioral, and sometimes supernatural details.
Physically, writers often describe changes in the eyes (dilated pupils, all-black irises, or uncommon colors), sudden changes in posture or gait, whispers or deepening of the voice, and unexplained bruises or markings that appear overnight. Sleep patterns flip—sleeplessness, nocturnal wandering, or rigid, unnatural stillness. Some stories give the possessed a cold touch or damp skin, while others highlight an odd scent (like ozone or rot) that follows her.
Behavioral signs tend to be more dramatic: florid mood swings, speaking in tongues or using languages she never learned, violent outbursts, unnatural strength, and startling knowledge of private things. There are also subtler shifts—a formerly kind character who suddenly uses cruel sarcasm, or a quiet person who becomes dangerously flirtatious. In scenes I love, these signs layer: a glint in the eye, a phrase in a dead language, then a sealed family secret spilled at 3 a.m. It’s the slow accumulation that tells you something supernatural is taking hold, and I get goosebumps every time it’s done right in stories like 'The Exorcist' or 'Chilling Adventures of Sabrina'.