Sometimes somebody can’t make it to a gig, so the horn arrangements have to change. Maybe there’s warning, maybe not.
I’m working in groups with two to four horns, often with doubling. For a lot of charts, two horns are enough; truth is, doing the arrangements, you start running into parallel lines and perfect intervals. You can do only so much clustering, after all, and two notes is enough to get the point across. In cases where I know one player is specifically likely to be unavailable—doubles on another instrument, or is the addition in the larger configuration of a modular band, or has some sort of life change going on—then from the beginning I’ll make the other instruments stand alone with that one fattening up the sound. That means tonics, sometimes octaves, fifths, fills, etc.
What’s more interesting is being the only horn on charts that normally have a section. Big-band style pops and tight licks are only cool because it’s a group acting as one. Otherwise it’s just me going splat at random times. So in those cases I’m going to do more freestyle, legato, out-of-rhythm variations on what the group would be doing. Where the standard arrangement is the horns doing a punch-and-fall on the downbeat, I’ll do a single legato note, shaped with dynamics, hooking into something the vocal or a rhythm instrument is doing. It’s good to enhance the various cymbals with a horn, like the bass drum meets the bass.
Another thing I can do is interact much more personally and conversationally with the lead vocal, since they’re also one instead of a group. Maybe I harmonize, maybe counterpoint, maybe echoes; these are all techniques that tend to be a bit clumsy if a horn section does them behind a single vocalist, though it’s wonderful antiphony when it’s vocal harmonies vs horn section. In one song, I played loose, legato triplets in the first two choruses, but for the last chorus and outro the harmony singer picked up her trumpet for a staccato syncopated horn section version of the same motif.
Point is, it’s a lot of freedom. That means more responsibility, but it’s also a chance to try out licks and gestures that I might make into horn section charts later.