Of course, Hollywood has given us the stereotypical view of how one behaves when possessed by a devil, or demon, as they often call it. Shown are all sorts of histrionics, aberrant and abhorrent behavior from the one possessed. You might hear guttural, evil-sounding voices emanating from the possessed one’s mouth, even a male’s voice coming from a female.
While all these effects may or may not be observed, I think they represent at best a very small portion of those actually possessed. It must also be remembered that Satan and his ministers can be very deceptive.
“And no marvel [i.e, marvel not]; for Satan himself is transformed into an angel of light. Therefore it is no great thing if his ministers also be transformed as the ministers of righteousness; whose end shall be according to their works.” (2 Corinthians 11:14-15).