Watch near the end of the movie, when everyone is at the Halloween party. Before everyone gets scared by the ghost, and you see the big shot of the people at the party, you see everyone about to scream and looking around and then they all scream and some scream late. It is really obvious.
They're all in shock.... it's a comedy element to the end of the film - they turn round, wide mouthed thinking 'oh god! a ghost' then they run - explains the delay...........