Comparing the work of Gil Evans or any other performance of a composition to a reciting the pledge is asinine. If we’re using spoken dialogue analogies, a more apt comparison is watching a performance of Hamlet. The words may always be the same, but the impact, context, and emotional content can vary wildly. The printed page, be it music or text, can only convey so much information.
Quality musicians reading from parts are absolutely engaging in the same kind of “conversation” as in improvised music; it’s more of a difference in degree of freedom. When reading music, you have to turn notes into musical ideas within the context of an ensemble all doing the same, like a conversation. Call it a conversation of interpretations. When improvising, you’re generally still confined by the chord changes and whatever substitutions you can imply, but there’s certainly more freedom. It’s still an interpretive conversation, just will fewer constraints.
Then there’s the reality that improvisations are often built on preconceived ideas and plans, not unlike a composition. If you listen to enough solos by the same players over the same tune, you’ll hear the same patterns come up and sometimes even whole solos almost entirely get reused.
Check out these two performances of Straight, No Chaser by Clark Terry and Bob Brookmeyer. These are two different performances with roughly the same solo by Clark Terry.
Then of check this one out with Clark Terry and Wes Montgomery from around the same time period. It’s a little bit different, but it sure does sound familiar, and not just the opening lick.
And just because his feel is so good, here’s Monk playing it the same tune.