The Taxi Driver mirror scene is oft-mentioned as an improv moment (and I don't doubt that it was improv based, as anyone who studied Meisner method repetition can attest) but that moment didn't come out of thin air ... it was based on a similar moment in REFLECTIONS IN A GOLDEN EYE in which Marlon Brando talks to himself in a mirror ...

Goodfellas didn't make the cut? C'mon...

Coupla' things about Kubrick. A lot of people thought he was a nutty control freak and too hard on actors, but I think his madness was justified, and he did what he did (especially annoying actors with endless takes) for sane reasons.

He spent years researching and writing the script for many of his later films, and years planning shots. He wanted to make sure all that effort was reflected on the screen (you can keep finding layers of detail in these movies). Why should he be bound to an actor's schedule when you and others put in years of disciplined work to get it to the filming stage? He was in a position to say to hell with that idea and he shot as much film as liked.

Moreover, if you think about something else he said you can see the sensible logic behind his method. He said that he was looking for something in the shot that would be unexpected (this after years spent thinking about what the movie would look like). In fact, screenwriters are often surprised that the best scene in the movie they wrote is not one that they had thought to be so important.

As proven by the improvisations on the list, it doesn't make sense for a control freak to allow actors such free reign. It does make sense for a director to get the best shot he can by whatever means.

Kubrick might have been hard to take but I think he was clearly following a labor intensive strategy to get the best that he could from what was available to him.