Alas, there is more than one secret. And they are all hiding in plain sight.
Here are a few of the secrets:
1. Technique with a metronome.
2. Theory, including harmony.
3. History, knowing the repertoire and the historical context from whence it comes.
4. And most importantly, ear training. Connecting the ear with the hands.
Known by some students as the boring bits. Know by professionals as the exciting bits.
It is the same for jazz and classical too.
If I can help, call me.