Improvisation in Film

I've come to realize that a lot of films that I really love involve a great deal of improvisation. Hou Hsiao-Hsien and Abbas Kiarostami use it in order to get natural performances from their actors (and non-actors). Eric Rohmer's greatest film, The Green Ray, relies heavily on the improvisation of Marie Riviere bringing her reality to Rohmer's plotting. Then we have the films of Jacques Rivette whose improvisations seem more heavily indebted to the theatrical tradition.

So what I've been wondering recently is what improvisation means on film. Can it really be distinguished from fully scripted and acted material? (This is something essential to Tati's second greatest film, Parade). In some sense the obvious answer is yes, as in that there are obvious cases of when someone is reading from a script. But beyond that, considering that a film essentially stays the same once it's made, are both improvisation and scripted acting just acting? I think this is an interesting concept to consider, the role of improvisation in film because I think it's quite a complicated matter, and obviously very different from what it means in theater.

Is improvisation just a means to an end, or is it something notable in the final product of a film?

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Mubi

Even though they may not be improvised, the films of Robert Altman, John Cassavetes and Henry Jaglom have a very improv-like feel to them. I do remember reading that large portions of the Ridley Scott film Alien were improvised as well.