Best acting in a monster/horror movie

[Dickey]

One of my favorite monster movies is the original 1951 "The Thing", starring James Arness. (Both remakes sucked, AFAIC). I'm sure there are many fans of the original 1951 movie, and what I noticed about the movie is how incredible real & natural the acting is,lots of ad-libbing, and some of the comedic element in the acting ("And I'll close the door"). Prolly the best acting in any movie I have ever seen. I read there is a name for this type of incredibly natural acting style, but it escapes me. Anyone else notice this, too?