I realize that as a (very rarely) working writer, my opinion on this is blasphemy, but actors can absolutely make or break a script. That’s why I love this list of ten well-known movie lines, all of which were improvised. Or at least not in the shooting script. Who knows what conversations were happening between the actors, writer, and director on set between takes. In many cases, the famous line in question is the only one anyone remembers. I wonder if the writers of these screenplays cringe when they hear a line they didn’t write quoted in association with a film.
The comments section has lots more juicy movie stuff.