I am starting to read about modal interchange.
Do modal interchange generally occur phrase by phrase in an orderly way (one phrase dedicated to this scale, and the next to another scale), or a little bit more chaotically like interspersed with a parallel scale within a phrase?
Short piece seems to suggest the latter is the case, but I just wished to know for certain.
Both can occur. In the example shown, the mixing happens within a phrase, but it also can happen that an entire section of a piece might be in one mode and another section in another mode.
It can, and does happen in any conceivable way. In all styles, and periods of music, moving from one parallel to the other occurs phrase by phrase, line by line, and yes, part bar to part bar. There are no 'rules' which dictate any particular places where modal or parallel changes should occur. If the music takes itself one way or the other, then that's what happens. Partly to add tension, partly to smooth things out.
