The best performances are an integrated combination of polished and casual.
I was lucky enough to see James Taylor and his band last night in a nearly 3-hour outdoor concert. He was singing songs from throughout his career, including many songs that he has performed for over 50 years. That, combined with his outstanding and well-rehearsed pro band, contributed the polish.
But in spite of all the preparation and experience, the performance never felt stilted or performative. It was fresh, welcoming, and casual in the best possible way. He laughed and joked, told stories, and in general made sure his audience was in the moment with him. (At one point, in the context of introducing he band, he acknowledged the moon that had risen – “The one and only! Keep being you!”)
There is a risk as your practice gets more honed – a risk that your performances get polished to the point of being lifelessly rote. The best performers, though, take their excellence as an opportunity to open up and find new life in the specific moments.