The central idea: a villain has made himself invisible. You don't know he's coming until he's got his fingers around your throat. And he is furious. The scene where a man who was forced to help him runs panicked into a town screaming, "The Invisible Man is coming!" is still disturbing.
Griffin's ferocious temper and megalomanic insanity, coupled with his invisibility, produces a frighteningly unpredictable package. He ends up with the highest body count of any of the classic Universal Horror monsters, as well as most horror movie slashers, most of which he does for no reason at all.
The death scenes themselves are increasingly nightmarish:
First, he strangles the constable nearly to death, and then finishes the job by bashing him in the head with a stool.
Later, while a search party is looking for him near a cliff-side, he takes the liberty of pushing two of them off.
He sneaks into a railway tower and bludgeons the switchman in the head with a lamp. It is left unclear if the man survives.
He then tops all of the previous crimes by derailing a train, which apparently kills a hundred people.
Finally, there is the death of Kemp, who he chokes, ties up, and then throws into a car, which he then sets in motion, resulting in a deadly crash.
Really, the entire film is made of Paranoia Fuel, as it milks the implications of Griffin's capabilities for all they're worth.
Not helping matters is the many instances of Black Comedy scattered throughout the film. Theoretically, this should add some levity to the film, but the resulting Mood Whiplash ultimately creates a sense of unease that never truly lets up until Griffin is finally defeated.
The care facility for insensates, people who lost most or all of their senses due to invisibility research. Particularly memorable is a man who keeps cutting himself with a shard of glass, implying that pain is the only thing he can feel anymore. When Darrien takes the glass away, the man desperately gropes around for it.
In "Tiresias," Hobbes is attacked by a Quicksilver-mad Darien. Hobbes tries to call for help from a phone booth, as an invisible Darien smashes the glass and tips it over before trying to strangle him.
In the pilot, Darien wakes up in the middle of night, feeling what later turns out to be Quicksilver Madness. It's pretty unnerving to listen to his description of what he's feeling while looking at himself in the mirror in the dark.
"It was as if someone was knocking at the backdoor of my brain—something that wanted in real bad. Staring into that mirror, I had the sudden, horrible feeling that another person was looking back—a stranger that had stolen my face."
An invisible bigfoot in a permanent state of quicksilver madness. Enjoy your hike in the woods.