Don't read this unless you've already seen the movie. There's some fairly heavy duty spoilers here.
One thing I totally forgot in trying to articulate just how random Hal's personality changes in during the movie is how Sinestro suffers just as badly.
For the brief amount of screen-time he has, Mark Strong is picture-perfect as Sinestro. The problem is that the script fails him completely when it comes time for Hal to prove himself and save the Earth from Parallax. At this point, The Guardians are so desperate for a means to fight Parallax that they have forged a ring powered by the power of Fear and are about to hand it to Sinestro when Hal shows up, says they can't give in to fear or use an evil power against itself. Hal then asks for a chance to prove himself and fight Parallax.
Suddenly, the arrogant but skillful leader, who has been expressing his doubts about Hal's ability from Day One suddenly is all "whatever" about the untested, incompetent rookie facing down a monster that killed most of The Corps best.
At this point in his life, there is no way Sinestro would stand idly by while innocents were in danger. Even if they were stupid, smelly Earthlings. Sinestro could have insisted that he stand with Hal in the fight to come, if only to honor his old friend Abin Sur. He could even have grabbed the yellow ring and flown off, decrying the Guardians' unwillingness to do what must be done.
But he doesn't. He only shows up once the battle is over, along with Tomar Re and Killowog, to save Hal after he nearly gets killed luring Parallax into Earth's sun. And at the end he says he was wrong about Hal and that they were fortunate the ring found someone so much like his old friend.
Which makes the bit after the credits where Sinestro takes the yellow ring from where The Guardians left it COMPLETELY nonsensical! Seriously. The only reason Sinestro was going to take the ring in the first place was because there was no other way to defeat Parallax - or so he thought at the time. Of course this begs the question of why The Guardians didn't get rid of the damn thing in the first place...
Getting back to Hal and the origin-story angle...
One of the biggest complaints I've seen in the reviews of this movie is that most of the movie is a standard origin story with nothing we haven't seen before in every other superhero movie. On the one hand, The Hero's Journey is a standard archetype for a reason, as anyone who has read some Joseph Campbell can tell you. On the other hand, it's a fair complaint... because Hal Jordan is probably the last hero who should get a standard Hero's Journey film.
Hal should already be a bad-ass man with a variety of heroic qualities when the movie opens. In fact, he is. It's only after being given access to power that he begins to doubt himself. Why? Because the Hero's Journey says he should doubt himself at the mid-point of his quest.
And I think that is part of what killed this movie for a lot of the fans - Hal is not a good fit for that story. Kyle Rayner would be. Guy Gardner, who had a lot of issues with his family that inspired him to heroism would be. And depending on which of his backgrounds you used, John Stewart could fit the bill too. But Hal? If you have to have a crisis for Hal, it should have been the same one as Tony Stark - the man-child learns responsibility.
Maybe that's why Hal was written so different in the middle part of the movie? Maybe the writer was trying to avoid accusations of "Oh, well this is just like Iron Man with the jerk learning to be less selfish." Just a thought...
And yet, the great irony was that in trying to make this movie more generic to the masses, I think it mitigated everything that would have made the movie stand out. Indeed, the biggest problem with the movie is I don't think it went far enough with the central concept: a group of alien space cops. The scenes on Oa are far too brief and the alien Lanterns far too underutilized.
Maybe this will be improved in the sequel... when there is one.