re: Movie conversations sound fake because nobody ever says "umm" or "uh"

1) Having actors talk at the same time and pause, stutter and stumble over words tends to slow down a film's pacing -- not the main goal for studios that want fast-moving, audience-pleasing movies. Altman and Cassavetes experimented with the technique and I'm sure some indie movies do it today, but they are never blockbusters.

2) Actors stuttering and interrupting each other also makes editing a film's soundtrack much more complicated (and therefore expensive). In Psycho Hitchcock had Anthony Perkins and Martin Balsam frequently interrupt each other, and Perkins stutter, when the detective interrogates Norman Bates (to illustrate Norman's nervousness). Some great takes resulted, but they took a long time to edit and added to the film's modest budget. Supposedly some takes that seemed great on the set could not be used because of soundtrack problems.

Hitchcock described drama as "life with the dull bits taken out". Truly realistic speech is never very dramatic.