Czerny described: "Beethoven's playing was incredibly powerful and full of character, marked by a matchless bravura and fluency; ..." "... Hummel's supporters accused Beethoven of abusing the piano, of having no clarity or purity, of using the pedal so much as to produce nothing but confused noise..."
A correspondent to the Allgemeine Musikalische Zeitung from Vienna 4/1799 described: "Beethoven's playing is brilliant in the extreme, but not very delicate, and at times becomes unclear ..."
Gerhard von Breuning saw Beethoven play a scale for him once and noted very curved fingers, so much so that they were hidden by the hand.
As a teacher, Ries describes him as "extraordinarily patient" "...he sometimes made me repeat a thing ten times of even more often." " ... If I made a mistake somewhere in the passage, or struck wrong notes, or missed intervals - which he often wanted strongly emphasized - he rarely said anything. However, if I lacked expression in crescendos, etc. or in the character of a piece, he became angry because, he maintained, the first was accident, while the latter resulted from inadequate knowledge, feeling, or attention. The first happened quite frequently to him, too, even when he played in public." [Wegeler/Ries p82-3].
"...he usually kept a very steady rhythm and only occasionally, indeed, very rarely, speeded up the tempo somewhat. At times he restrained the tempo in his crescendo with a ritardando, which had a beautiful and most striking effect." [per Ries p94]
From memoirs of Sir John Russell in his "A tour through Germany 1820-1822 [B.Nwsltr v8#2p40]: B., believing himself alone began to improvise at the keyboard of a friends with others in another room of the friends home: "Having been left alone, B. now sat down at the pianoforte. He began by playing some short, isolated broken chords, almost as if he feared being caught at some mischievous deed. But gradually he began to forget everything around him and lost himself for about a half hour in a fantasy, the style of which was extremely varied and which was especially characterized by abrupt transitions. The connoisseurs were enraptured, and for the uninitiated it was all the more interesting to see how the music from this man's soul expressed itself in his face. He seemed to have more feeling for the bold, the imperious and the tempestuous than for gentleness and yearning. His facial muscles swelled and his veins bulged. His eye, which already seemed wild, rolled about intensely, his mouth twitched, and Beethoven had the appearance of a sorcerer who felt overwhelmed by the spirits which he himself had conjured up."