My question: If in the middle of writing a contemporary romance a secondary character takes on an entirely different demeanor than you intended, and he becomes larger than life, the tone darkens and switches to what could be romantic suspense, do you toss that character out of your relationship/love story or do you go with it and expand his character and go back and foreshadow?
Although I never follow this advice, I really do believe you don’t change anything in a first draft until the draft is done and you can see the shape of the whole story. I think the smartest thing to do is follow the Girls to the end and then step back and see what you have. But you’re right in that a change in tone breaks the promise made to the reader at the beginning of the book, so in the rewrite you’re going to have to pick a lane.
The key is going to be what kind of story it is at the end, what happens in the climax. The end of the story determines everything else in it. You can’t have a lighthearted comedy where the dog dies in the end. You can’t do a slapstick ending to a dark plot. (That actually happened to a very good movie, The January Man, a story about a serial killer and the serious hunt to capture him that ended with Kevin Kline rolling down the several flights of stairs with the killer and making wisecracks the entire time. It was awful.) So at the end of the first draft, look back and see what you ended up with, and then see how that outlier character fits in. If he’s a betrayal of the climax, you might be better cutting him or at least making him lighter. But if he’s the match for the climax and the beginning just starts too light, rewrite the beginning. What you’re looking for is unity, that sense the story is all one piece, all the smaller pieces locking together and matching to make a whole.
Standard Disclaimer: There are many roads to Oz. While this is my opinion on this writing topic, it is by no means a rule, a requirement, or The Only Way To Do This. Your story is your story, and you can write it any way you please.