Back to Miller's JUSTICE LEAGUE, the movie ended up costing millions without even a proper start. Miller compromised to filming in Canada, which required a completely new production schedule and led to numerous other issues. It didn't take long for Miller to drop out, the actors to be let go, and the entire project to be scrapped.
But WB still wanted their own cinematic universe to compete with Marvel's. And for that they needed their own Iron Man, and that's when their eyes turned back to Green Lantern, who had become very popular in the comics recently. Greg Berlanti, Michael Guggenheim and Michael Green wrote the script, with Berlanti also set to direct, and the movie was greenlit for 2011. However, Berlanti soon got dropped for lack of experience, as WB feared he wasn't cut out to handle a big-budget blockbuster in his first gig as a director. They instead settled for Martin Campbell, who did it for the paycheck and didn't even bother to hide it.
Ryan Reynolds and Blake Lively was cast as the leads per studio demand, although Campbell favored Bradley Cooper and Rose Byrne, and the movie was given a $200 million budget that quickly ballooned up due to their dumbfounded decision to make the entire suit CGI, which proved far more costly and complex than they had anticipated.
Add to that a lot of on-set tension, as Campbell made sure to voice his dislike for Reynolds and demand numerous takes of most of his scenes because he just wasn't Bradley Cooper, dammit. Reynolds was understandably pretty sour on the whole thing by the end.
Nevertheless, they were so certain of the movie's success they greenlit not only a sequel, but also the next chapter of their DCCU, THE FLASH, both written by Berlanti and Guggenheim, and with Berlanti once again in talks to direct THE FLASH.