I have, above all, repeatedly pointed out the purpose of all forms: a layout which guarantees comprehensibility. I have then shown what are the conditions that go with comprehensibility; how it is a question of the kind of listener one is writing for (and, in so doing, defined the difference between light and serious music...); how there is always a manifest relationship between an idea's difficulty and the way it is presented, so that an idea which is hard to grasp demands a slower and broader presentation than does one which is easy to grasp; the role played here by tempo, so that when the notes move quickly, things must unfold more slowly. How, for example, when the harmonies are hard to grasp, the tension must be lower in other directions--and other things of the same kind. Obviously one cannot formulate this kind of consideration of material without psychology, since the material is destined to affect the psyche and only comes into consideration at all through this function.
--Arnold Schoenberg, Style and Idea: Selected Writings, p. 316.I'm not sure anyone else has expressed these things with the same degree of clarity.
Bartók knew something about form. Here is his Piano Concerto No. 3 with Martha Argerich:
UPDATE: While we are on Bartók piano concertos, this is a pretty interesting performance of number 3. The soloist is Jean-Efflam Bavouzet with the London Philharmonic Orchestra, Vladimir Jurowski, conductor. Notice how the conductor cues the hard-working pianist as well as the orchestra. Also, they have moved the percussion from being in the back to being in front, level with the solist. Good performance.