I'm not exactly sure what aspect of the 'horn section thing' the question is
directed at, but as a foundation, all of the players (if there are no charts
involved) must know the chord structure and the note that comrpise the chords,
of every tune. The section needs to be able to instantly harmonize in as many
chord voices as there are players, what ever the chord structure is at any
given time. In the group I play in, the original tenor player takes the  lead
in determining what the rhythm figure will be for any spot in the song, then
the other two of us have the responsibility of creating the harmony. It is
actually very easy if each player knows where they are in the chord structure
of the tune. This process does require good ears and thorough understanding of
the material and the chord structure. Though it may be easier to work off of
charts, the way we do it allows infinite possiblilites on on-the-fly
harmonization.  No practice necessary.

The other comments regarding the 'feel' of the section along with dynamics, etc
also apply.

RC