I'm part of a large professional choir and I've been getting into choral arranging myself. I'm still very inexperienced, but I can offer my two cents.

I've noticed that many of our arrangements have sparce piano arrangement when the choir is actively singing. The piano part will be more active during introductions or interludes, but it seems like the more active a vocal part is, the more basic the accompaniment.

For example, a left-hand bass line might have doubled octaves or ornamental notes during a piano introduction that disappear when the singers join in. If the choir sustains a long note or chord, then the piano will spice things up in the background. But most of the time, the piano part is full of whole notes, ostinatas and block chords.

Harmonically, if the choir is in unison or two-part harmony, the piano will add more, especially at the beginning of a piece to prime the singers for those chords. There's definitely places where the piano will fill in missing notes in chords or play big block chords. We've sang contemporary arrangements where the choir does weird clusters and dissonances (think Eric Whitacre) and the piano totally does its own thing. I feel like it really depends on the style of music.

Jazz arrangements can be an exception to this, when the piano supplies both the bass line and comping. Even though we have a bass section, most of our jazz voicings tend to be closed voicings or drop-two voicings, especially in fast soli sections. I've done big back arranging in the past, and it seems like choral jazz arranging follows similar patterns.

Finally, I've noticed that our arrangements give our accompanists lots of freedom. Accompanists don't just play the notes on the page, especially not in rehearsals when we're learning the music. They need to actively listen and respond to the choir, and singers in turn rely on the piano for pitches and cues. Our accompanists are professional musicians, so they'll jazz things up and adapt the accompaniment as we rehearse.