General Principles

First and last rule: There are no rules here, only observations. If I can help you to make them your own observations, and you find something worthwhile or useful, it is yours forever.

Golden rule: Never undervalue your own creativity, in either the sphere of practice or interpretation. Make everything your own. Get right to the inside of yourself and the task you have set. Evaluate and learn, in each unique opportunity, from as many different approaches as you can think of.
Feel it all as a part of yourself.

The Primary Principal in Performance will be to portray your own Landscape.
Specifically this is outside the scope of this book, but generally, by respecting the composer's wishes and trusting his annotation, you will find the direction. Add to this the uniqueness of YOU, in the moment, and you have a powerful recipe for a performance Landscape . . . the impression that the composer and you desire to leave with the listener.

The Primary Principal in Practice is to absorb, as efficiently as possible, by using awareness and the intellect. Absorption takes place during repeated exposure of the awareness to the object to be absorbed. It is absorbed into the whole being of the Pianist, the emotional pianist inside the understanding pianist inside the body pianist.
The process is successful to degrees all the time, even unconsciously, but must be continued until the object of absorption can be performed at will without too much apprehension, and can fulfil all the musical criteria in context.

I feel strongly that the contextual consideration is the most important and therefore it is wise to study the landscape first, away from the piano and then zoom in, as it were, to the object, remembering these first observations regarding rhythmic and dynamic flow.
(We shall look at these two aspects of The Science in a further portion of the book.)

Strive for contrast. Yes - the very name of the instrument hints at this - piano (soft) forte (loud) and the colours of the keys act as a constant reminder.
Although majestic or delicate, rich or sweet, the piano, because of its intrinsic mechanical nature - (there is quite a lot of clever mechanics between the fingers and the sound) - nevertheless cannot do one or two things that perhaps other instruments can.

For example - once a note is played - there is absolutely nothing you can do to affect the volume. You cannot crescendo (grow - get louder) nor can you diminuendo at will - the note will simply deacy, naturally until released.
Nor can you add any sense of vibrato to emotionally heighten the sound - as you can on wind instruments and stringed instruments. The tone produced will be absolutely even.
You do not even have to breathe (apart from staying alive, that is) so the sense of phrasing that wind players, brass players or singers can apply by simply breathing is not necessary to actually play a pianoforte - the phrasing has to be interpreted, at first consciously, rather than instinctively.

To compensate for these missing abilities I feel it is vital to always strive for as much contrast in playing as is possible.
Contrasting dynamics - loud to soft.
Contrasting articulations - staccato to legato.
Contrast or balance between different musical elements - melody - accompaniment.
How much can you heighten these contrasts - how clearly can you mark one extreme from the other - how much of a range can you give to these matters of contrast?
All questions for you to explore and experiment with.

Above all, I would consider the fact that music is about relationships.
Sound in relationship with silence.
Pitches in relationship with each other - rhythms in relationship to a metre.
Timbres - colours - textures in relationship to each other.
Contrasts being another aspect of relationships.
The relationship of composer to performer - the performer to the listener.
And finally - critically - the relationship of the pianist - with him or herself.