Many voices at once
A choir adds size and emotion that a single voice can’t: a held “ahh” under a drop, a chant in a film cue, a gospel stab in house music. Choir samples come in two kinds. Sustained notes and vowels, often multisampled, let you play your own chords. Recorded passages of real choirs give you a performance to chop, stretch or reverse.
Getting a natural sound
Choirs are recorded in large rooms, and the reverb is part of the sound. Use a matching space for the rest of your arrangement, or shorten tails with a gate. Layering a choir pad an octave below a string section is a classic cinematic trick.
Recordings of old choral works can be confusing licence-wise: the composition may be public domain while the recording is not. Each card’s verdict is about the recording, which is the part you need cleared.