OK, what’s our job? Basically it’s to take a melody the composer gives us and present it to an audience for X number of minutes so that it satisfies the style and engages their attention. Plain and simple, if they (the audience) know what’s coming next, you’ve hit the “yawn, when’s the next song coming?” wall. You’ve failed! I don’t care if it’s a Catskill show (am I dating myself?), underscore for a chase scene in a “straight to video film,” or an orchestral accompaniment for Shirley Horne (we should all be so lucky).
Most professional musicians (studio & otherwise) have the playing background to assess what’s going on in the charts in front of them. When you can satisfy their musical needs for innovation and freshness (usually it’s a unique jazz session or outside film/TV session) you’ve got a winner. Face it these musicians have seen it all and it takes quite a bit of creativity to grab their interest.
Remember what Hank (Mancini) said, “The song is the thing and the arranger’s function is to make it memorable…” How do you make it memorable? You add as much musical variety as possible using all of the devices necessary to do it. The key word here is necessary! Many beginning arrangers try to use every trick in the book when a few would be more appropriate. For instance, modulation. This in its self would be enough to create variety, there’s no need to re-harmonize the bars before & after. It detracts from the effect of the device you’re employing.