I've recently begun studying subplots, and I've realized I don't know how far I should develop them. Does a subplot need everything a main plot does? Does there even need to be a conflict for the subplot (if it's short), or can it simply be a few scenes that link together to show something related to the main plot?
Before I go any further, let me explain that I develop my novels before writing them. This means I break down exactly what I need in the novel, and how to achieve it. One of the things I do is work with the main plot, and develop the main conflict so that it is complex and cannot be easily resolved. I'm wondering how much of that one does for a subplot.
How much development does a subplot need?
Additionally, do I need separate stakes for a subplot (assuming the stakes for the main plot don't apply to it)? What about additional characterization? My question isn't limited to simply developing the plot.
Note: I'm assuming that the answer to this question will depend very much on the size of the subplot in question. If that is the case, please provide answers for several different sizes of subplots (ie, if the subplot is only a few scenes, you generally do A and B; if it spans three books, you want to do C as well).