What are your favorite circumstances that drive the plots of horror/suspense films?Some of mine are:
* ISOLATION - Such as in NIGHT OF THE LIVING DEAD. A handful of people in an isolated house, surrounded by zombies. Other examples would be ALIEN, THE SHINING, and MISERY.
* PARANOIA/CONSPIRACY - Such as in INVASION OF THE BODY SNATCHERS. You don't know who your friends are. Another great example is ROSEMARY'S BABY.
* SEEMINGLY INDESTRUCTIBLE VILLAIN - Such as in HALLOWEEN, ALIEN (again) or THE THING.
What are some others?
For me it would be the isolation factor as well as the frightening creature aspect. Both of which you mentioned.
BEING STALKED -- knowing there's something out there trying to catch you. You hear a noise, get ready to hit the villian, and then the damn cat sticks its head up.
Many (most) movies combine all two or all three of these elements.
The thing being a perfect example.
"THE WORLD ISN"T WHAT IT SEEMS" - Matrix, Extistenz, Dark City, The 13th Floor, etc..
Getting chased by the killer. No matter what the characters do the killer is right behind them or just out of site. Then you add in the fact thye are usually out lost somewhere so they really don't know where to go.
The unknown. Forces beyond our comprehension invade our nice, ordered lives, or an unseen presence, of unknown nature, cohabits with the characters, watching them without their knowing and making strange things happen. Poltergeist, the Changeling, Amityville and many Lovecraft stories make use of this. Unfortunately, to make it effective requires a subtlety not found in most horror movies, and few filmmakers can resist the urge to explain too much.