To be able to grasp the substance of the music itself is to be willing to begin a never-ending search. The task of the performing musician, then, is not to express or interpret the music as such, but to aim to become part of it. It is almost as if the interpretation of the musical text creates for itself a subtext that develops, substantiates, varies and contrasts the actual text. The subtext is inherent in the score and is itself boundless; it results from a dialogue between the performer and the score, and its richness is determined by the curiosity of the performer.Barenboim is, of course, writing from the standpoint of a solo musician (like a pianist, which he is) and a conductor (which he also is). There is another full set of elements that come into play when the members of a group of musicians add their own subjective and often un-uttered versions of a musical subtext. Musicians all have different reactions to form, dynamics, and harmonic changes, and they all have technical concerns. Everyone has to consider the difficulty of particular passages, and everyone has places in the music where they need to blend, and places when they need to come to the musical foreground. There are also concerns with intonation, counting accurately, and playing the desired articulations with the desired degree of comfort.
Perhaps the only constant in any given performance or reading is the unchanging role of the (usually dead) composer. If the composer is alive s/he would probably prefer to play a constant and unchanging role in a performance too, and simply be a fly on the wall. At least I feel that way. I believe that a composer's responsibility for making choices, sub-textual and otherwise, ends when a piece of music is played "in the flesh."