Why should a pianist who already plays the demanding and advanced piano literature, such as Chopin, Rachmaninoff, Beethoven and Brahms, still take piano lessons? And if they would benefit from lessons, what type of teacher should they be seeking?
Many advanced players would greatly benefit from lessons. We all have habits and patterns of which we are unaware. Recently I heard a pianist who has quite a bit of experience play a Chopin Nocturne. It had a repeating pattern in the base. Every time she played this pattern she had a slight delay from the first note to the second. Maybe she was totally unaware of this. Maybe she did it intentionally as a "nuance." However, when you do something meant to be an expressive nuance over and over, it is no longer a nuance; it is expected, annoying, and derails the rhythmic integrity of the piece. She also used the soft pedal for well over half the piece, even parts that weren't meant to be that soft. Was it intentional? Or did her left foot just get stuck in a pattern she has no idea she has? This pianist desperately needs a teacher who can hear and point out these things. More than that, the teacher would need to be able to do much more than say "don't do that thing you are doing." The teacher would need to guide the student into other ways of expressing herself, to a mastery of technique which would allow her a far wider range of expressive tools.
With my advanced students, my approach is simply this: as I listen to their playing, I am acutely aware of every small detail which disturbs my listening, my enjoyment, and my perception of the beauty of the piece. These "disturbances," therefore, are the problems with the playing. It doesn't mean they have to play the piece in the same way I would play it, for many different playings of the same piece can all be beautiful. But I am listening for something that undermines or unbalances the piece at a very fundamental level. At the same time I am watching everything they are doing physically, from hands to arms to torso to feet. I am not watching the written page, as I've seen almost every teacher do, to make sure they are playing the "right notes" (my ear tells me that). Through my years of experience, with my training from my earlier studies with a master teacher, and my own playing, I can see exactly what is happening physically that is causing the musical problem. It is rarely a situation that they "decided" something should sound such-and-such a way; it is almost always something that they do unconsciously. Unfortunately, we all fool ourselves into thinking we have decided on a certain interpretation of a piece, but in reality it is the limits of our physical mastery that has often decided it for us. I work with my advanced students to develop the mastery, the tools, to enable them to "say what they have to say" through their playing. I don't teach "interpretation," that is, I don't tell them to play this part softer, this part louder, this part slower or faster. If I did, they would be playing my interpretation, not their own.
When a disturbance occurs, I have many tools and methods to get them to a different physical way of playing that passage, which gives them a new experience. With a new physical experience, they begin to hear differently. I believe it is a profound truth they we play the way we hear and we hear the way we play. If you play jerkily, you will hear jerkily, so to speak, so it will sound normal, even pleasing, to you. Think about an absolute beginner who plays Twinkle Twinkle, pounding out every note. He or she thinks it sounds fine. As they develop physically and musically, their playing changes to be more subtle. So it is as every level; the more subtle and deep your listening is, the better your playing. And vice-versa.
Apparently Tiger Woods still has a teacher, even though he is considered to be at the top of his profession. Does the teacher win more golf tournaments than his student? No. But he can see things in the student's physical movements, down to tiny details, that affect his playing of the game. Seeing them is the first step to changing and improving them. It is a wise person who knows that there is always room for improvement and does not rest on his laurels.
If you are an advanced player and feel there may be aspects to your playing that you are not entirely happy with, you may decide to seek out a teacher. If you find a teacher who can only have you play hours of technical exercises (which you could do on your own), or if they only point out the dynamic or phrase markings in the score (which you can see for yourself), or if they play the piece for you in an attempt to "show you how to play it" (you can probably find videos of greater pianists if it were only a matter of copying what you see), you are with the wrong teacher. Find out about that teacher's earlier studies: did he or she have a master teacher? Does the teacher still play the piano, at least for his or her own enjoyment? (This tells you whether he/she works at solving problems and improving their own playing in the ways they are suggesting to you, in other words, practicing what you preach.) Talk to other students of that teacher to find out if they feel their playing has improved under that teacher. Just as important, are they enjoying the process? If you aren't loving the lessons and loving the process of growing with the music, look for another teacher. If you are pouring your attention, heart and soul into your playing, you deserve a teacher who does the same when he or she sits next to you at the piano. Don't accept less.