Christopher Walken made the difficult trick shot to win the match against Tara's (Eastwood) boss on the first take. He was supposed to make a "trial run" for the scene, but he asked that the cameras go ahead and roll, in case he happened to make it on his first try, and he did.
In real life Mars Callahan is an accomplished pool player who met co-writer Chris Corso in a pool hall where each was trying to hustle the other. After a bitterly contested game (neither will tell who actually won) the two became best friends. Swapping war stories about their mutual experiences playing pool, the two decided to write a script based on their experiences and observations. Two weeks later the script was completed, but it would take another ten years to get it to the big screen.
There were no camera tricks or special film editing used for any of the billiards shots in the film, although many of the special "trick shots" (other than the one above) were performed by billiards legend 'Robert LeBlanc', who appears as professional "Bobby LeBlanc" in the film.
The "quiet as a church" line at the beginning of the movie was a slightly altered line taken from the movie "The Hustler", a film starring Paul Newman about a pool hustler.