One of the iconic ghostly images is that of a spectre walking through solid walls.
The explanation is simple, the ghostly image is conformed to the past, and in the time it came into being there was no wall at the point

A famous example of this is from York where ghoslty Roman Soldiers were seen marching through a cellar - see opposite

Since ghosts are traditionally associated with traumatic events in life or the moment of death, it may be powerful emotions play a part in this process.
This might be at death or during life, see the case study of 'a ghost named Philip' which underlines the human aspect behind ghosts.