I think the main thing is TOTAL immersion. I find that, as long as you listen to ONLY the genre you wish to get your head around for a considerable length of time (weeks if not months!) you quickly gain an ear for what to play, and more importantly, what NOT to play.
But dilute your immersion by listening to a lot of other stuff, you tend to lose that filter.
To a certain degree, it’s possibly more important to learn what not to play, or at least to recognize the minute you play something that isn’t idiomatic and make a mental note to never play it again, and I think that degree of sensitivity only comes after you steep yourself in a genre to the exclusion of all others.
If you don’t restrict your listening to other genres you get to hear licks or changes that are not idiomatic and they tend to influence you subconsciously.
It’s definitely a PITA to be that focused for a couple of months or weeks, not to mention the effects on those around you who have to put up with your monomania (!) but I honestly feel it’s the best way to absorb a genre you previously didn’t spend much time with…
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An arranger is just a tool. What matters is what you build with it..!