play music without working out the fingering

There was a pianist ,at a church I used to go, who could play any music given her. I saw soon after she was given the music, she simulated the playing sitting on a bench outside the sanctuary with the music on her lap. I did not see her marking the music. Apparently, she didn't need to chart out the fingering. When she went into the sanctuary, she played for the choir; it was flawless at least to my ear. I am sure someone at this forum can do just the same. If you are one, could you tell me: What technical preparations does one need such that he can play without having to worry about the fingering?

 

That's really interesting.