When I was watching the show in the 60's and 70's, I remember one moment that made me lose sleep.
It was when David and his blonde (girl) friend (can't rmember her name) were in the dark room developing pictures and I think it was the ghost of Gerard who appeared above them - or something like that. It's been awhile since I saw the scene so some of the details are a bit vague, but I sure remember tossing and turning quite a bit that night.
I could have sworn that he was looking at the picture with Carolyn...*searches*
Aha, I found it!
From DarkShadows.com (synopsis of episode 1077):
[spoiler]Daphne appears to Quentin, then later to David and gives him a model boat. Daphne leads Quentin to a room which has a unfinished letter that is incoherent. Carolyn is still upset about her husband's death. Carolyn, Maggie, Quentin and David go on a picnic and David takes photos. When David develops the photos he feels there is a strange figure in one of the pictures; Gerard Stiles watches Carolyn and David looking at the photograph.[/spoiler]
I think the scariest moment for me, that I can remember off the top of my head, may seem odd...but oh well.
It would be when
[spoiler]Carolyn was staying at Sky Rumson's house, and Jeb was lurking outside her room, and all you could hear was his breathing...very suspenseful scary.[/spoiler]