I was wondering what the standard procedure for transcribing an orchestra piece for a solo piano. Obviously, the large number of parts being reduced to a single instrument results in certain restrictions that need to be taken into account, etc. So I was wondering what sorts of techniques are used when doing such a task, and the resulting effects on the final transcription. :)
I have considerable experience in transcribing for the organ, less for the piano. All the same: I doubt there is one standard procedure here. Two musicians will surely reduce an orchestral work differently. I'll offer some comments on how to begin, but it feels pointless to write much here: I believe the best advice is in the last section of my post.
Firstly, what is the goal of the transcription? How faithful should it be?
At one extreme, the transcriber may remain so faithful to the printed score that no additional notes will be introduced, even if they might convey the spirit of the music more effectively. At the other extreme, the arrangement is so free that the word transcription ceases to apply. This decision will affect what technical devices are available.
For transcriptions at the faithful extreme, you have many (most?) two-piano reductions of piano concerti. These are easy to find, check them out and compare with the corresponding full scores, for example on imslp. Generally, art is not the goal here, but function. These transcriptions must be playable without deep preparation by accompanists, as they find much of their use in rehearsals and in the earlier stages of competitions.
For transcriptions at the other extreme - although the orchestra isn't involved - Busoni's transcription of the Bach Ciaconna is a very good example. All the resources of pianism available at Busoni's time were used. Art was the goal here.
Transcription as interpretation
Sometimes you can reach a deeper appreciation of music by transcribing it. This is because transcription forces you to decide what is more important and what is less. Platitudes like "every note is crucial" must be thrown away as you get your hands dirty making compromises. Clearly, reducing an orchestral score for piano requires leaving things out. But what?
Organize the lines into a hierarchy of importance: Where are the main lines? (Melody, theme, etc.) Which instruments are adding rhythmic punctuation? Which instruments are adding harmonic colour? (Following the melody in thirds etc.) Which instruments are merely doubling?
Translate idioms using corresponding techniques. Here is where your knowledge of the instruments comes in.
One typical example is how to handle repeated notes in the strings. Usually, piano transcriptions will render these with tremolos. The effect is different, and there is a (dated? pedantic?) school of thought that such tremolos are unpianistic, but they seem to be the best translation of the string idiom. They preserve the pulsating character.
Sometimes you need to rely on the player. For example, orchestral climaxes are often achieved by adding brass. Typically you hear fifths. An experienced pianist will know that bringing out the fifth in a chord can suggest the brassy orchestral effect.
Learn from the best
If your goal is art rather than function, then why not take Liszt as your teacher? imslp has his solo transcriptions of the Beethoven symphonies. It would be difficult to find better guidance. Compare his writing with the full scores to understand the techniques at your disposal.