Most conductors are pianists and string players, and probably only a minority have specialized on a wind instrument. To rehearse effectively, a conductor needs to understand the character of each instrument, what it can and cannot do, so he doesn't lose credibility by asking for the impossible.

The greatest players can play very loudly and very quietly, but they don't usually produce their quietest playing until they know you expect it. All instruments have limitations: it's hard to play softly in the lowest part of the double-reed instruments (oboe and bassoon) or in the highest part of the flute and piccolo. You have to know how far to go in your demands.

As you study a score, you need to form a clear idea of what kind of attack you want from the wind instruments in each spot and whether you want the notes played long, short, or in between. It's usually obvious, but occasionally you'll need to change the way a particular passage is played.