Basically, a joke backfired on me. A long time ago, pretty much in the Dark Ages in Internet Time, someone made a crack about Trixie running off with Regan. (She also said something about Jim and Miss Trask and no. Just no. Not even I will go there.) So, I thought, well, despite the way he acts, Regan really is only 22 when the books start. It's not *entirely* out of the realm of possibilities. Or taste. I wrote a story that was eventually incorporated into A Simple Kind of Fear. (The original version of the episode now called Falling, The Director's Cut, Part 1.) That was *supposed* to be the end of it. My joke on the person - she knows who she is - who threw that bit of insanity out there.
Up until that point? I was totally Team Dan. Sorry, Jim fans! I know you are legion, but Dan's the Man. Or he was. After I wrote Falling, I could not get Trixie and Regan out of my head. They took up residence and refused to leave. I wrote the first six episodes of ASKOF - Madness - and then kept right on going.
And that's pretty much the whole story. The idea grabbed me and won't let go. Please note, though, that there's nothing really hinky going on in ASKOF. Trixie is 17 when she falls for Regan and 18 before anything serious happens. In The Cold Lake Murders (an entirely different universe), she's 25, though in the interludes there are hints that she's had Regan wrapped around her little finger since high school.