it really is kind of interesting how attached people are to the idea of the improvised line and the genuine reaction in film. nearly every single famous moment or line has some sort of claim floating around that it wasn't in the script and was either made up on the spot by an actor or was sprung as a surprise on the actors to get their 'real' reaction. you know, sometimes people do come up with good lines in advance and write them down, and sometimes people can pretend to be have a convincing emotional response on screen
also important to note that a lot of the 'improvised' lines that people talk about are things that might not be in the script, that the actors cone up with while shooting, but still discuss and figure out with the director before shooting the final takes. genuinely improvised lines without any prior knowledge are pretty rare. i think instances when the people involved in making a film collaborate to make changes to a script are really cool, and the fetishisation of the 'genuine reaction' and 'improvised line' also cheapen this form of 'improvisation' i.e. collaboration and flexibility among artists
