It's pronounced "ensemble" by the way. And, one way that a pianist would have a role would be by being in the background as an example and playing his part. He's going to play what he needs to play. The question does seem a little to vague, but truly, I would say simply, that the person who is playing it, just plays his part as they all make music sound good.
Those are scales. the highlighted notes (aka the guide tones) are notes in that specific chord that give a harmonic pull towards the next chord. generally they are the 3rd and 7th note (that’s what determines what type of chord it may be: major, minor, major-minor, or dominant). they are the most harmonically important notes in the chord since they determine the quality of the chord itself, so make sure they are in perfect tune.
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