How long should I hold focus on a character who will turn out not to be the main Antagonist? I am thinking about using a character as a decoy. Someone to point the finger at but then revel that they are not the problem and is not the person of interest. I do not know how long I should hold focus on this character before the MC realizes that he is a decoy. I am thinking abut midway through the book but I also want to give the real antagonist time to develop at a interesting character.
Agreed. Can you find a way to have the real antag introduced and developed without implying that they'll be the antag? Depending on how your story is structured, the real one could be there from the start, or at least early on, and be made interesting but sympathetic or neutral. You could even play it up as a betrayal when they show their true colors, if that serves the tone. In something I'm working on, the true antagonist is basically offscreen for most of the story and her sisters are implied to be the villains, though her existence is a frequently revisited point and there's information laced throughout that she may not be as sympathetic as she appears (unreliable narrator, woo). It's not actually until the last quarter that she finally shows up in the flesh and turns out to be the big bad, and her sisters were just trying to keep her contained. Granted, my story is partially a mystery, so it's fitting. That may not be so for yours. But I'd definitely see how much development you can give the real antagonist before the reveal, so you don't feel lie you have to cram as much characterization is as possible n a short amount of time. Ditching the red herring too early makes them feel a bit pointless, but it really depends on your pacing whether midway would be too soon or not. Midway seems sort of early to me, but I don't know your plot.
Developing two characters--the antagonist and the red herring--parallel from each other will serve you well. Based off your post, I don't know whether or not the actual antagonist is a known known in your story an unknown known; that is to say, do the readers know the antagonist through narration, or do they not know him through narration but know he's out there? If it's the former, then develop the two characters so the twist has a bigger impact. If it's the actual antagonist is an unknown known, *SPOILERS* I'd watch something like True Detectives season one as an example of this being pulled off rather well.
Have you seen the movie A Perfect Getaway (2009)? It pretty much does what the OP describes in his story, and does it well in my opinion.
