A performance and a recording are not the same thing, not as I see it. I can perfectly well record a whole sonata by bits and pieces, some today, some tomorrow, and then glue them together. Is that a performance?
Now I see what you mean. I forgot about the editing aspect. Ideally we should be able to give a complete start-to-finish performance in a single take, especially if the piece is short. But where random mistakes are inevitable and editing is an available tool for getting rid of them, then one might as well take advantage of it. I'm not so sure it's a good idea to spread activity over several days. In the case of a sonata with separate movements, yes, OK, because in a way the movements are different pieces. But you wouldn't want to spread the recording of just one movement over several days, would you? It makes more sense to just play it in a single recording session. When you notice yourself making a mistake, you can either just carry on as if nothing had happened (which is what you would do in a live performance), and then play it all again and hope you make different mistakes this time; or else you could just stop, backtrack a bit, and carry on (hoping not to make the same mistake again), all without stopping the recorder. Then you can cut out the mistakes in your editing session later. This is much better than splicing together fragments which were recorded on different days, because in the end you want to produce a recording which "could have been" a one-take performance, and unless you work with click-tracks (which are the devil's own invention) you'll never get tomorrow's tempo to be the same as today's, and tempo isn't the only thing that will be different.
For me the performance is the way the musician interprets the work.
I only partly agree with this. For me, interpretation is a matter of feeling, crassly put it is how you think
it should go, or how you want it to go or wish it would go. Performance is how it actually
goes, and is subject to any accidents which get in the way of translating your mental artistic image into a physical rendition.
We should be able to judge if a recording has artistic merit and then, and only then, point out the flaws.
I think opinions are divided (and rightly so) on that. Often the flaws are so prominent that what merit there is is beyond recognition.
A great teacher is not necessarily scarce with praise, but he will praise what is good and then attack pitilessly what he sees as wrong.
I'm not convinced that is a good teaching technique. I think the "attack" should be less pitiless, less destructive, it is after all a tool for improvement, not a weapon of destruction. And the praise should be less glowing, we should generally avoid superlatives. Let's reserve praise for when things are really
good, not for when they're just "sort of OK". Praising that
is harmful. Praising (or at least recognising) improvements, is important, though, especially where previous criticisms have been acted upon.
What should you do? I believe you should first offer an overall review: is it expressive? Is the pianist tranquil? Is he too agitated? Is his technque up to the challenge? Is it musical? Than yes, by all means, point out any errorrs that you detect, because at this jucture, he knows where he stands as far as the interpretation goes and can concentrate on polishing the mistakes or deciding to rehaul the piece.
I'm not sure it's quite as simple as that. Praise is a good reward for improvement, but is perhaps out of place for stuff that is just sort of OK at the outset. And praiseworthy musicality really wants to shine through before it's praised, and it can fail to do that if it's obscured by technical problems. Eddy is right here; the technical stuff needs to be conquered before very much is done on the musical side. I think the notion of "knowing where he stands as far as the interpretation goes" and then concentrating on getting rid of the mistakes is the wrong way round. It is not before all the mistakes have gone, and you are technically on top of things, that you are free to concentrate on what really matters: the artistry.