Some quick possession tips off the top of my head:
- Speak in multiple voices/tones/accents/volumes. Don't be erradic, but maybe show the demon speaking as the girl, and the demon speaking as the demon.
- Create controlled chaos in your movement. bring your energy from 1 to 10 then 0 followed by an 11 that slaps people in the face! The trick here is to seem unnatural, like… (bum bum baaaaa) a demon is in control.
- Think of interesting twitches, adopt one, and then take it over the top. Clicking your teeth, playing with your hair, twirling your tongue in moments of silence, a wrist or finger that seems to be unable to rest… Don't take all of them, just pick something to carry as an underlying note of demonic creepiness.
- If language isn't an issue, the demonicly possesed tend to have a touch of poetic potty mouth syndrome. Put some curse words together with begnign phrases and say it with conviction. Thats IF you can get away with it.
I might be able to think of some others but this is just what came to mind
Hope it helps.