The Joys

So I'm 38,000 words into the first draft and a character is introduced who, I realise, really needs to be trailed earlier in the story in a Chekhov's gun kind of way.

Then I realise that the back story of this character is important because particular aspects of it add complexity to her reaction to my protagonist and events. Of course this is true for all characters, but more so for this one (this novel is written 1st person, and the protagonist has no knowledge of the new character's back story).

Then I realise that the back story of this character could plausibly be a spin-off in its own right.