We did Blue Moon for an encore number in the last show I wrote, but brought a guitar player on stage to make the fifth voice as it seems easier to pull it off. If you have a crooner for the solo line, the Bass part is very easy to copy if that is what you want to do, and the inner voices pretty much follow the 1st inversion harmony I mentioned. When we are learning a song, first and foremost is to get the harmony right so you don't have parallel 5ths (if you do it will sound strangely hollow at that point and you will need to correct it) But to get back, we do it with whole note harmony to find trouble spots. Then do it very slowly acapella, then start adding rthe nuances of the song.. That stuff is easy to deal with. WHen you talk about a part wandering around, it is normally the bass part. If you watch a bass player, he will sometimes walk from chord tone to chord tone and in the process will pass through notes that are not at all in the chord. Leading tones, passing tones, etc Your vocal bass can do exactly the same thing. He just needs to know when he is on chord notes and when he isn't If this is confusing let me know. But for example the bass part when he is singing Blllue Blue Blue Blue moon.........dip di dip di dip, he is walking through a line where only half of the notes are truly in the chord.