Eastwood is remarkably sure of himself and self-disciplined, trusts his actors to do their jobs right the first time, and works with the same crew time and time again until it has become a well-oiled machine.
The most important bit of this is the last part though, because Eastwood has a remarkably experienced crew that know him and each other so well they make productions run smooth as silk. He’s a good boss to work for, his crew is highly talented, has good morale and camaraderie and they enjoy working together.
(Eastwood directing. It’s a whole lot more about his crew than about him personally, though.)
Sure, Eastwood might be known by actors for filming a single take and moving on, but the real secret to his success is the behind the scenes teamwork. Warner Brothers, which has been behind most of his productions, loves him for it.
He’s worked with the same editor, Joel Cox, a staggering 63 times over the years.
A stuntman he used a lot, Buddy van Dorn, worked together with Eastwood 45 times.
Allan Robert Murray from his Sound Department and Donald Harris from his Music Department collaborated with him 35 and 34 times respectively.
The list goes on and on.
“Eastwood’s favorite collaborators” page at IMDB is littered with crew who’ve worked with Eastwood well over a dozen times and most run into the dozens.
So yes, Eastwood generally brings a film in on time and on budget (even occasionally under budget), but that’s not just him being self-disciplined and self-assured. He just has the best crew in the business.