My relationships with possums have not been congenial. We had one in the big tree in the backyard. It would go up there and sleep, I guess. One day my husband was painting some truck parts on the lawn. He was sitting on a stool and the possum fell out of the tree (who knew this could even happen?) and almost fell on him. He jumped up and the possum got up shook himself and wandered off. I kept the dogs in that day so we wouldn't have any confrontations and that night, there he was back in the tree. It took a while to convince him that he needed to live elsewhere.
We also had a young possum come in the doggie door that opens into the master bath. One of my dog woke me at 3 am on a Sunday morning growling and staring at the bathroom. I went in and there he was, sitting by the doggie door and looking a bit freaked out. I tiptoed out and closed the door hoping the he would just leave. I waited half an hour and when he hadn't left , I woke my husband. It took my husband a good 1/2 an hour to get him back out the doggie door. The bathroom door was closed so I'm not sure what went on but there was a lot of growling and hissing involved.