This was almost "Under the Hood: Green Lantern", because the movie is so obviously flawed that I spent most of my time trying to figure out how to take the skeleton of this film and use it to make something actually good. It's a movie that encourages you not so much to watch it as red-pen it and send it back to Warner Brothers with a note saying, "No. Wrong. Do it again."
The flaws are enormous and fundamental, starting with the protagonist. They made a strange and distracting choice in this one, grabbing pretty much every single character flaw from Hal Jordan and piling them on one after another while adding a few new ones just for this movie. As a result, I couldn't tell you what Hal's emotional arc is in this movie if you put a gun to my head: Is he trying to overcome his fear of commitment? Is he trying to show everyone that he's as good as his father? Is he trying to cope with his fear of dying like his father? Is he trying to overcome his fear of failure? Or is he too much of a loose cannon, with the arc being that everyone else (including the Corps) needs to learn that he's right and they're wrong? The movie offers absolutely no answers and never really wraps up any of these things; in the end, Hal mutters the Green Lantern oath under his breath and then defeats Parallax in less than five minutes. It was as if they assumed that Hal would be sympathetic solely because of Ryan Reynolds' degree of dudebro roguish charm, and so they didn't need to do anything at all to make you like the guy based on the script.
Oh, yes, and Parallax. Let's have a long, sad talk about Parallax. Look, I know he's a major element of the last decade's worth of Green Lantern mythology, but...he's not really a villain. He's not even really a character. He's a walking collection of plot kludges that solved problems that the writer was having in the 'Green Lantern' titles at that time. He has no clear motivations--is he trying to destroy the universe? Rule it? Eat it? Punish it? Never explained. He has no clearly defined powers--in the film, he's just a big cloud of smoke with yellow souls in it that swoops through Coast City, except when he's a yellow goo that turns Hector Hammond into a creepy monster-person who otherwise has sweet Fanny Adams to do with the rest of the film. (And by the way, I'm going to give a little tip to you, filmmakers. If you do, in fact, plan to have Hector Hammond as a villain so you can retcon in a lifetime personal connection to Hal and give them the parallel problem of being unable to live up to their famous fathers...maybe have them meet sometime before the halfway point? The late second act is a little too late to have them bump into each other and start chatting.) And speaking of Hector Hammond, at least he had a connection to Hal, even if it was just made for this movie. Parallax could be pretty much anything from a giant meteor to a plague of space frogs for all that it matters to the main character.
And they have Sinestro in the movie, but they don't use him as a villain, because "you need to set it up". No, no you don't. Sinestro is an ex-Green Lantern who decided that the Guardians were soft and fear would exert control and curb disorder far better than "willpower", and so he turned to the power of fear to do the job that they could not. That's a sentence, not a movie. What people generally mean when they say that you need to set up Sinestro's fall is that they feel like Sinestro becomes a stronger villain when he's also Hal's mentor, but that's not necessary, just desirable. Given how weak Hal's Rogue's Gallery is outside of Sinestro (maybe Krona, maybe Star Sapphire, maybe the Manhunters but they're basically Sinestro without the ring, but before long you start getting down to dregs like the Black Hand and the Shark) you have to open with something big. Sinestro is your biggest gun. There's no point in saving him for a sequel that may never happen. Not to mention, if you do for some godforsaken reason want to use Parallax as a villain, you use him after you use Sinestro. Sinestro is a mortal using the tools of a god. He's less threatening if Hal has already beaten the god in question, even if it was through one of the dumbest and most awkward Chekhov's gun moments in cinematic history. "Here, poozer, let me teach you one thing and one thing only. The best way of defeating something large and monstrous is to throw it into the sun. Remember that in about an hour or so, okay?"
And even if you are going to set up Sinestro, maybe you should then decide to, oh, I don't know...set up Sinestro? As it is, Sinestro's emotional arc is the only easily comprehensible one in the entire film, and it's "arrogant and prideful hero wannabe learns that real courage isn't just power and combat skill, it's facing up to your fear and defeating it." It is an arc that means the one thing he's not about to do is put on the freaking fear ring at the end, by definition. Putting him in the neon yellow CGI animated bodysuit at the end (and I could write an entire post on the way this film was utterly drunk on CGI, using it for things that would have been done far better as practical effects like make-up and costumes, but life's too short to spend the rest of it detailing every single way this movie sucked rocks) was the one thing he should not have done based on his character as shown in this movie. It wasn't just an unearned Big Moment, it was an anti-earned Big Moment.
I could go on further--the opening saga voiceover was a pointless infodump that was covered later on in the movie in its entirety, there was no effort made at establishing the Guardians as actual guardians of the universe that people would listen to (the seemingly endless number of Guardian heel-turns only ever worked because they first grounded the Guardians as examples of the Wise Mentor archetype and then showed that they had a dark side), Abin Sur still had a spaceship even though there have been no less than two stories written in response to the question, "Why is a Green Lantern flying a spaceship anyway?", there's a criminally good setup line for a heroic quip that's utterly wasted (Parallax says "You are nothing without the ring," which absolutely begs for Hal to do something awesome and say, "No, the ring is nothing without me," but instead his sorry butt is saved by Carol launching cruise missiles at Parallax)...but I think I've made my point. There are exactly two things this movie did right, and one of them was not succumbing to the urge to call Tom Kalmaku Pieface. This is not a good track record.
I think a good 'Green Lantern' movie could still be made. In fact, it's been made pretty easy. Just look at every single creative decision this movie ever made, and do the exact opposite.