Here are few points I have learnt while working and researching on sub-plots.
1. A sub-plot involves lesser important characters or what we call secondary or supporting characters. The significance of the events in a sub-plot are also of less importance when compared to the main plot.
2. A sub-plot should connect or be related to the main plot in some way: the characters involved in the sub-plot may have a vested interest to sabotage either the protagonist or the antagonist’s agendas.
3. A sub-plot can run parallel to the main plot without intersecting or affecting it at all. But these sub-plots have to be powerful by themselves and should involve characters who are important enough by themselves. The actions of these characters should affect the story in some way.
4. A sub-plot can intersect the main plot at regular intervals when the characters involved in the sub plot feel that their interests are being affected by either the protagonist or the antagonist’s actions. These characters can have hidden agendas of their own, which though the protagonist and the antagonist are not aware of, yet it affects them in some way. It requires a lot of planning to create sub-plots that constantly cut into the main plot.
5. Sub- plots take up less action and the conflict from the subplot is milder when compared to the conflict in the main plot. The conflict in the sub-plot has to be interesting and has to affect the story in some way: directly or indirectly.
6. Many experts advise that good stories should have two subplots intersecting the main plot at regular intervals. When these subplots cut into the main plot, then the conflict is intensified; this increases readers' attention and keeps them glued to the pages of the book.
I feel I am a sub-plot novice. I am completely clueless where sub-plots are concerned. I have been looking a lot for resources and tips to help me with sub-plots. While outlining I try to plan the details of atleast two sub-plots. What about you all? Do you all pay a lot of attention to sub-plots? How do you plot and plan the sub-plots which are extremely important. We all would love to learn and get these details from you.