Spotnitz, the brains behind Anderson's long-running sci-fi TV hit, wrote the title character, secret agent Sam Hunter, with Anderson in mind after she mentioned her dream of playing a female version of Matt Damon's famous action hero Jason Bourne.
However, Spotnitz was left "heartbroken" when Anderson had to pull out of the project.
He tells Britain's BBC Radio Derby, "Originally it was Gillian Anderson. She actually wanted to play a female Bourne and I wrote this with her in mind and she was hugely helpful actually in developing the script and had millions of ideas, and she's just an amazing actress, who I know well obviously from The X-Files.
"But the process took so long and by the time I actually had the green light she couldn't do it. There were other things that she was committed to, which was heartbreaking and left me with this terrible problem of how to find somebody else who could play this very demanding acting role."
Spotnitz went on to audition hundreds of actress and eventually hired Australian star Melissa George for the part, and he insists he has no regrets over the casting.
He adds, "We looked at 200 actresses in London and New York and L.A. and Sydney. For me, Melissa was the only choice. As soon as I saw her, I realised she was the right one."