What Could Have Been: Green Lantern.
In 1997, Warner started to become interested in developing new franchises based on DC Comics characters, and Green Lantern was one of their first picks. They asked Kevin Smith to write a Script for a movie about the character, but Smith refused, partly due to the Superman Lives fiasco he had recently been through. With Batman & Robin's failure, Warner scrapped the idea of a "serious" Green Lantern movie and started looking at the project as a potential comedy.
In 2006, Robert Smiegel wrote a The Mask-based Script intented to be starred by Jack Black. Adapted from "Emerald Dawn", the Script was focused on Judd Plato, a furniture store employee who is bestowed the Green Lantern power ring by a dying Abin Sur after eating a dead coyote on live television and taken to planet Oa to help the Green Lantern Corps fight their greatest enemy, Legion. Other classic characters from the comics included Sinestro, Kilowog, Tomar-Re, Katma Tui and Xax. The project was eventually scrapped due to negative feedback from the fans.
In 2007, Corey Reynolds offered himself to write a Script for a movie centered on John Stewart and able to spawn a potential trilogy that'd introduce Hal Jordan, the Green Lantern Corps and the Justice League of America. He also intented to star on the movie. His spec Script, Green Lantern: Birth of a Hero, was well-received by Warner and even scouted for a potential 2010 release date as a Spin-Off of George Miller's Justice League: Mortal, which would feature Common as John Stewart. Columbus Short was also approached for the role, but turned it down.
When Justice League: Mortal fell apart, a solo Green Lantern movie started to interest Warner's executives again. Greg Berlanti, a big fan of the character, presented his idea for a Hal Jordan-centric trilogy, using concept art portraying David Boreanaz as Hal Jordan and Carla Gugino as Carol Ferris, to convince Warner to greenlight a Script. They did, and, in 2009, Berlanti, Marc Guggenhein and Michael Green turned over their first draft, also based in "Emerald Dawn" and featuring a Parallax-inspired Legion and Hector Hammond as the villains.
Warner decided to produce the movie and, after entertaining the idea of asking Miller to direct it, they chose Berlanti for the job. Geoff Johns was also asked to do uncredited rewrites, with later drafts being based on "Secret Origin" and featuring Parallax and Hector Hammond as the villains. Berlanti eventually felt that the project was too big for him to handle and, after scouting the likes of Quentin Tarantino and Zack Snyder, Warner hired Martin Campbell to replace him as the movie's director. Michael Goldenberg then joined production, rewriting sections of the Script alongside Johns. The search for the cast then begun.
Several young actors were rumored for the lead role of Hal Jordan, including Ryan Gosling, Chris Pine, Sam Worthington, James Franco, Jake Gyllenhall, Anton Yelchin, Brian Austin Green and Shawn Roberts. However, the final candidates were Bradley Cooper, Jared Leto, Justin Timberlake and Ryan Reynolds, who got the role.
For Carol Ferris, Rose Byrne, Michelle Monaghan, Eva Green, Keri Russel, Jennifer Garner, Diane Kruger and Blake Lively, who got the role.
For Thaal Sinestro, Hugo Weaving, Jackie Earle Haley, Geoffrey Rush and Jason Isaacs were considered before Mark Strong landed the role.
For Abin Sur, Doug Jones was considered before Temuera Morrison got the role. Likewise, John Cho and Adam Beach were contenders for Tom Kalmaku before Taika Waititi was hired.
The rest of the cast, composed of Peter Sarsgaard, Tim Robbins, Jon Tenney, Jay O. Sanders and Angela Basset, was chosen without many bumps on the road.
The roster of characters, on the other hand, was very long, with John Stewart, Guy Gardner and Kyle Rayner and even Clark Kent being considered for minor roles and/or cameos. Eventually, John Stewart got the "part", and former marine Nick Jones plays him in a small scene in the film. The original Green Lantern, Alan Scott, on the other hand, was originally the mysterious governmental agency that connects the stories of Jordan and Hammond. He was replaced by Amanda Waller, played by Basset, and the Department of Extranormal Operations (DEO).