André Bazin: The Realist vs The Expressive
André Bazin was an influential French film critic and film theorist: he started to write about film in 1943 and was a co-founder of the renowned film magazine Cahiers du cinéma in 1951. He is mainly known for arguing that realism - as opposed to expressionism - is the most important function of cinema: his calls for objective reality, deep focus, and lack of montage are linked to his belief that the interpretation of a film or scene should be left to the audience.
This opinion opposed the popular film theory of the 1920s and 1930s, which emphasised how cinema could be a medium for manipulating and escaping reality.
As illustrated in the drawing above, German Expressionism and Soviet Montage fell into the expressive category, so Bazin didn't like these. Cinema Verite falls into what Bazin described as realist: long takes, straight editing, naturalistic... kind of like an observational documentary, but just not a documentary.
As you can see with the snakey timeline, cinema has naturally progressed from more widely expressionist in the 1900s (with the cinema of attractions and slapstick Hollywood comedy) to perhaps being more aligned with realism in the 1930s and onwards (narrative-driven cinema, and what has come to be known as classical Hollywood style).