Yeah... I would say they take it allright if no one 'else' sees the antagonist- but they /audience/ has to get some kind of GLIMPSE of it...

And if you draw it out too long, they can get annoyed- at the very least, a shadow, footprints, some indication of IT, should show.

In the movie "Paranormal Activity", you never 'see' the ghost/demon/whatever... but you DO see manifestations of it- and even then some people are vaguely put out that there's not more.