I have known a couple of people who could see angel and demons. They both described seeing demons as a colored (any color) shadow entering and exiting people they were talking to. It manifest itself as a abrupt change in the possessed person's personality. Have you ever been talking with someone and, all of the sudden, their mood changes drastically for the worst, catching you by surprise?
According to these two, angels never possess anyone and usually pop up randomly, starting after a traumatic childhood experience. One says they look like tiny, albino adults with wings and harps, non-descriptive faces that fade away as he grew older. The angels would come alone or in a group to deliver a message, instruction, or a warning. One swears up and down if it were for their intervention, he would have been dead many times over!
As to why they were having these experiences, neither can answer that. People have told them they were possess, evil, etc. Someone even suggested that they needed an exorcism. One actually talked to a priest who dealt with exorcisms. After the priest described what could happen and clarified to the guy that, yes, he could die, he decided to deal with it.