According to hadith, it is possible for there to be occasional interactions between the living and the deceased. For instance, the deceased may occasionally (with the permission of Allah) come back invisibly to check on those whom they left behind. It is also possible to communicate with the deceased in a dream (and for it to be a real conversation). Sometimes this can help the grieving process. There are some narrations about the Imams speaking with the deceased.
Also, when it comes to people with a very high level of faith/spiritual development, such as the Prophet, Imams, and the genuine martyrs, it is not unusual to receive some guidance from them or have some communication with them after they pass on. For instance, before leaving Medina, it is related that Imam Husain (A) had an intense vision or dream of the Prophet (S) who predicted what would happen in Karbala.
However, usually, when people are experiencing "ghost" things around them (seeing things, hearing things, other stuff), it is due to jinn. Jinn can mimic the deceased and have knowledge of them. The deceased really have better things to do than hang around scaring people!
In many cultures, there is a lot of fear of ghosts, but there is no reason to fear human beings who have passed on. Most people hurt each other in this life due to bodily needs and desires (money, power, lusts, anger, jealousy, etc), and none of that is relevant after someone has passed on and no longer has control over anything in this world.
The future of the the deceased is in their new existence (in the barzakh) and their afterlife, and the best thing to do for them is to pray for them or do acts that might benefit their soul, such as charity on their behalf. If for some reason someone is concerned about a deceased person being upset, it certainly doesn't hurt to pray for them, recite Qur'an for them, and do good acts on their behalf.