Paraphrasing a random Schumann quote -

[opus10no2]

'The eyes should hear and the ears should see'

Meaning, when a learned musician sees a score, he hears the music from his inner ear. Also when he imagines music in his head, he should readily be able to imagine the image of the score.

This ability, along with a couple other things(obviously co-ordination..), most notably- great *flexible* experience with many pianistic figurations, is the key to great sightreading.
Also it's the key to starting as a composer too.

I personally think the printed score is a wonderful invention, and for all it's restrictions, it allows for a kind of mapping out, a canvas of sound, and even the greatest of ears are aided by the eyes of a musical architect.

[pianistimo]

i like that quote.

what do you think one can do to increase their ability to 'see with their ears?'  listen to a lot of other pianists?  or, do they imagine sound away from the keyboard just like most pianists do 'memorizing notes.'  i usually do this whenever i see a score - but when i am away from the score - i start thinking about sections.  rarely, if ever, do i go through the entire piece in my head - really relaxed - imagining it as i would like it to be entirely through.

perhaps a mistake of students is too much sectioning?  after a while - you come to expect section one to end and section two to start.

[opus10no2]

It comes with time from experience with sight-reading, working with scores in general, and having a sharper perception of pitch and rhythm.