Jennet has complete power over the children she controls and drives to suicide, the Jump Scare of the movies may seem cheesy, the way the solicitor's daughter just stands and stares as she immolates herself certainly wasn't.
The stage play has plenty, but here are some moments:
The dream sequence the Lawyer goes through.
The first time we hear the coach crashing.
The Lawyer and Spider, his dog, nearly dying near the marsh.
The final line of the play invokes Fridge Horror. It indicates the Woman in Black's ghost was really in the performance, and the actor's daughter is next...
The nights in Eel Marsh House Arthur spent.
Every single time you hear the rocking chair and music box, you know something terrible is going to happen...