I find this pretty amazing (I have been listening to Gould, and reading about him, continuously since 1978, but only came across this a few months ago):
Gould is rehearsing Bach, (mostly) unselfconsciously, on a rather battered and uneven-actioned piano (at his parents' holiday home).
At one level it is an enjoyable, albeit corny, 'genius at work' kind of thing.
The performance is, I suppose, too fast; but what I find fascinating is the idea of what he was aiming at - especially when he tries the same passage several times.
A detached, crisp style, independence of lines - but one where everything is phrased - even the trills!