WHAT MAKES A SUBPLOT SUCCESSFUL?
Choosing a subplot begins with choosing characters with which to work. Who among your secondary characters is sufficiently sympathetic and faces conflicts that are deep, credible, complex and universal enough to be worth developing?
If none are to be found, it might be worthwhile to grow some of your secondary characters, depending on the nature of your novel. Do you intend it to be a sweeping epic? If so you certainly will want to construct a cast with plenty of subplot potential. Is it a tightly woven, intimate exploration of a painful period in one character’s life? In that case subplots will only pull you and your readers away from the main purpose. You may not even want to clutter your novel with multiple viewpoints.
Subplots will not have the desired magnification effect unless there are connections between them. Thus, the main characters in each subplot need to be in proximity to one another; that is, they need a solid reason to be in the same book. Therefore, in searching for subplots, I recommend first looking to those characters already in the main character’s life: family, classmates, friends and so forth.
One of the most difficult subplot tricks to pull off involves creating story lines for two characters who at first have no connection whatsoever, then merging those plotlines. For some reason, this structure is particularly attractive to beginning novelists. While such a feat can be pulled off, again and again I find that novices fail to bring their plotlines together quickly enough. Beginners often feel the need to present scenes from each plotline in strict rotation, whether or not there is a necessity for them. The result is a manuscript laden with low-tension action.
A second requirement of subplots is that they each affect the outcome of the main plotline. Subplots widen the scope of the novel’s action, but if that is all they do, then, once again, the result is likely a sluggish volume.
A third quality of successful subplots is that they range. In 19th-century sagas this often meant ranging high and low over the strata of society, from princesses to beggars, from the palace to the gutter. Social scale is a bit harder to pull off today. More helpful, I think, is to portray a variety of experience. Your setting may be restricted to one milieu, but ranging over that milieu in all its aspects will enrich the world of your novel.