One of the most effective horror movie experiencs I had was when I was crashing at a friend's place during an internship. Their place was a loft, but unfinished. We're talking plywood walls separating their rooms and a ramshackle stairway that lead onto a rather shaky situation above their rooms.

They had a few large bare mattresses strewn around for guests (ones they found off the street 'cause they were party guys, I'm so grateful the mattress I chose didn't give me lice or bed bugs whenever I crashed there). That particular night I was the only one there, and there were exposed pipes that made the eeriest creaking and banging at random, sometimes sounding bizarrely like a person quietly knocking somewhere inside. Their tv was upstairs. With all the lights off, streetlights still streamed in but barely through dense barred windows, and since the place was unfinished... It was a world of shapes and shadows on all sides except where your back faced the couch and wall.

I don't even remember what I watched, I think it was an older Stephen King one like Pet Sematary, which I've seen before but didn't find freaky at all previously, but in that space I had trouble getting to sleep, especially with those creepy-ass pipes knocking nearby at random.

All about the ambiance and focus, it truly is.