I recently attended a chatroom "brainstorm session" on Iwrite network. I was trying to get my MC's storyline figured out for a YA fantasy I had an idea about. As we chatted, one thing one of the other ladies asked me is if I had a character arc for the female secondary character. Well to my embarasment I had to admit I didn't. As we discussed her arc everyhting about the story seemed to fall into place including the questions I was having about the MC.
Well to hammer that idea home, I just started to exchange chapters with my new crit partner (they're awesome) and I needed to revise some of my chapters for my 2nd MC in that story. At that point in time I realized that her character arc was also absent. (see a trend here?) Not the female aspect, although I do write males better than females. LOL The problem i was having was in the fact that I wasnt addressing the character arc or structure of the secondary characters.
Now let me clarify, if the character is truly secondary and only has a minor role in the story just addressing that characters arc and or what the character wants to happen or needs to do is probably sufficent. If for example the character wants to ABC and it doesnt interfere with the main structure then let it play out. But don't introdouce a character (even a minor one) and their motive if you don't plan on resolving it.
If the character is either the antagonist or another MC you should have some structure planned out. I have even found that with Brook's style that any or all of the MC's: Antagonist, protagonist, second MC, (or love interest) can have diferent plot points mid points and pinch ploints although PP2 is most likely the same and where all of the structures come together. Although you can have one structure for all of them too, but I feel that seperate structures leads to a deeper plot and less flat characters.
The main structure of the book however will follow the MC's structure and is the "recipie" you follow in respect to the theme and concept.
For example, If the MC's character arc involves him becoming used to living a new style of life then his arc should reflect that. The main structure will highlight the story points that acomplish that structure. If the other MC's arc takes them through a process of them going from confident to being a victim and then becoming confident again that arc should address that AND that characters structure (pp1, mp and pinch's) should highlight parts of the structure that pertain to their arc. If the antagonist's arc involves them deciding that they want something regardless of the consequences, their arc and structure will reflect that line.
You will treat all three arcs and structures as seperate but cohesive lines with all three arc and structures coming together to colide in some awesome PP2 3/4 of the way throught the book.
Wow I hope that was clear and concise and not resembling the mud my kids were playing in this afternoon. Please feel free to comment and ask questions if you want/need.