Spirits of people passed away are different than ghosts. You can talk to the spirit of dead people anytime you want and you call them to you. They will always hear.
When I have 'channeled' spirits for people I always find it interesting how they continually say "Tell him/her that ...." Like the spirit didn't just hear what was said or I need to be there for them to tell the person how they feel. I needed to be there to give the response only.
For those who disbelieve. I didn't believe it either. That is the hardest part of getting good at it is doubting oneself. I have had a large number of what I consider proofs to know that I am not just crazy. Though I am crazy in a number of other ways.

Ghosts and spirits have a sense of humour. One told me his name was "Tom" then he said "Dick" of course "Harry" was next. Strangely enough it seemed to prove to the client that her father was really there. I just say what I hear.