- Don't compete for the melody line: As solo pianists we attend to the melody with love and care. When the melody is doubled on both the solo instrument and on your part and you bring out the melody, you will make your soloist nervous.
For example, your singer will over sing if you emphasize the doubling of his line; your flute player will barely make it to the end of the phrase if you decide to take extra time to reach the high note; or your cello player may play out of tune because you played the line straight through.
When the score doubles the melody, put your attention on the harmony, and shape the chord progression to match your soloist's phrasing.
- Adjust your dynamics for the soloist's lines: When you play on your own, the decision for dynamics and phrasings come from your own music line.
But when you accompany, your line might call for a crescendo just as the soloist' line will get to a lower register; if you started you crescendo then, the soloist's line wouldn't be heard at all.
Other times, you'll see pp markings on your part while the soprano is supposed to hit that high note; if you played pp then she wouldn't be able to hear the harmonic support from the piano that she needs to be confident.
Playing with someone is not what makes you a collaborative pianist, but playing with your soloist's needs always in mind does.
Picture from http://www.musolife.com/piano-accompanist.html