Equilibrium/WMG

Preston barely got away with going off-drug.
As soon as Preston stops taking Prozium, he starts acting emotional, rearranging his desk, rescuing puppies, and groping handrails. It seems like this is exactly the kind of behavior the police should be watching for, and Preston is hardly discreet. The only way this makes sense is if making sure  and that  that event where Preston killed a squad of officers over a puppy.
 * Isn't this actually referenced in the movie?
 * It is. The only other way what was said makes any sense is if, which would require a Gambit Roulette of utterly unbelievable proportions, since Preston initially went off the drug entirely by accident.
 * Accident? Preston had been subconsciously rebelling ever since the death of his wife. See below.
 * I think a good chunk of his escaping notice is that people on Prozium can't notice emotion in others as effectively. Empathy is as much emotion as skill, and Preston had enough of it before going off that he noticed his first partner thanks to the inflection on one sentence. The little kid in the beginning who was ordering guards to detain possible sense offenders from a crowd would be another such prodigy, but everyone else is probably unable to notice. Therein lies the trap Prozium represented to Libria: you're giving people a drug that makes them ignore people who aren't taking it except for a rare few who can potentially "feel" so strongly even the Prozium doesn't completely deaden them. That said, Preston was really not good at hiding emotion, which is understandable since he'd never had to.
 * Are you going to be the one to question the most talented Grammaton Cleric around? Maybe there are some other normal citizen sense offenders around this corner here...

DuPont and the 'elite' of Libria don't take Prozium themselves.
They're not even subtle with this. Everybody else lives and work in sterile, functional 'environments' with purely utilitarian furnishing, all identical. One of the primary duties of Grammaton Clerics is to seek out and destroy relic artwork, because it might tempt people to feel. And then we see that DuPont lives in a palace with statuary, paintings, priceless antiques, even silk wall hangings. We also see Brandt (Taye Diggs' character), DuPont's henchman, openly indulging in several emotional behaviors -- pride, ambition, anger, even gloating. Then again, one of the classic perks of being an Evil Overlord is that rules are for other people, so it's not surprising that Libria turns out to be just another corrupt dictatorship where the leader and his chosen few hoard all the wealth and privilege to themselves - only this one also uses propaganda and drugs to make the masses not care about wealth or privilege. All DuPont and his cronies need to do is keep the masses ignorant of what their private lifestyle really looks like.
 * According to the Director's Commentary, this is exactly the conclusion you're supposed to draw.
 * The director also said that they wanted to make DuPont's office more lavish to make the point obvious, but they ran out of money.
 * Alternative theory: the leaders need to have emotions to provide drive, creativity, and the full range of possible decisions with an abstract goal in mind. If everyone was a drone, Libria could stagnate like a machine that's never attended to.
 * It doesn't hurt that DuPont and his underlings appear good at keeping up the pretence of being on Prozium.
 * This one is pretty much a given. In DuPont's pleading for his life at the end, he states that he feels.

Prozium is the most powerful tranquilizer ever made.
Xanax times ten, or a highly efficient SSRI.

Preston was being conditioned to go off his meds at the right window of time to infiltrate La Resistance.
He was a high-level cleric whose wife was executed for an EC-10 violation. She had probably already gotten to his kids, and all they would have to do is trigger him to seek out the resistance after going rogue in a suitably eventful way that didn't kill more than a couple dozen of their people. The only reason he dropped the capsule in the first place was because of some trigger in the morning news, and he didn't get the capsule replaced because of the same. The Prozium makes people susceptible to such conditioning, as well as or because of suppressing emotion.
 * She had gotten to the kids. Preston's son and daughter mention they stopped taking Prozium completely when their mother was taken away. They wouldn't have done that to begin with if they weren't already on a reduced dose.

Clerics get shot all the time.
The "don't be there" gun kata ability fails with a perfectly normal statistical rate, it's just that nobody cares notices or is told about about a dead cleric that isn't their partner or a family member, and it's incredibly rare for a cleric to outlast more than two partners.
 * Hard to care about ex-partners when you've got all your emotions chemically supressed.
 * Alternately, clerics don't get shot when they are shot at because there are enough pre-planned events to give the Clerics a nigh-superhuman reputation. If you keep hearing about how a Cleric took out 35 people without a scratch today, then 24 the week before and 52 the next week, you are going to want to avoid them like hell. Then when you do come face-to-face with a Cleric you are too pissing-your-pants scared out of your mind to aim well anyway.
 * The reason Preston is never shot is because he simply happens to be on the far end of the probability curve-displaying a limited form of quantum immortality: while movie!Preston is never shot, there are thousands of alternate universe versions of him that are, and he somehow (perhaps through some kind of superpower) always manages to be the lucky one.
 * That's actually used in Terry Pratchett's Dark Side of the Sun.
 * There's basis for this in the movie without being outlandish; Preston isn't sure he can actually make the hit on Father, or even succeed in a suicide run. This means that the performance he gives in the lie detector room and down the hall of mirrors is atypical.
 * Another option: most clerics don't actually get into gunfights. They come across as mainly having senior administrative roles, and when they do go into the field they command entire squads of sweepers with machine guns and APCs. Their martial arts training means they can defend their reputation against sweepers and other challengers in the training room. Preston is just a Crazy Awesome exception to the norm, which would also explain why he's so overpowered at the end of the film.

Prestion is a Time Lord.

 * It makes sense! He was able to avoid all those bullets and kill beat up all those soldiers because he'd already been through it once or more!

The lie-detector technician is a sense-offender.

 * Which is why Preston spares him; he can sense it just like always.
 * Makes sense... the guy's "oh shit!" fear response isn't quite the way of Prozium.
 * That more falls under the heading of "Prozium takes out the highs and lows, not the whole thing." The Sweeper in a similier situation earlier managed a "Fuck!" before being shot. The strength of the I'm-gonna-die feeling is a tad too strong for Prozium to supress.
 * I was under the impression that Preston spared him because he was unarmed.

The Resistance was holding back.
Every sense offender stronghold we see earlier in the film is taken down quickly and easily. They can't seem to hit even one damn Sweeper, the only ones that die are killed by Preston. Then at the end of the film the Resistance fighters take down whole groups of cops without any trouble at all. They were holding back anyone in their group who could actually fight and sacrificed the others (with or without their knowledge) so the government would think they had more of an advantage than they did. Also explains the problems with the Mona Lisa being on canvas etc: It's a fake. A good one, but a fake to throw to the Clerics and make them feel like they won big.

The plot is what happens when Marijuana is legalized.
When it was finally able to be over-the-counter, everybody hit it...including philosophers and politicians. To a pothead, thinking everybody should be as placid as them seems like a good idea. With the current drug trade, a drug would be so potent to keep the subject as placid as heck before WWIII. And thus, when it actually hit the world, society's standards of art became so low that traditional art was not cutting it. And thus, the media war began.

Eh, it's the only way to make sense of this movie full of Straw Vulcan.

Prozium and the Effectiveness of Gun Kata
It's hard to imagine a drug that suppresses emotional responses not having other side effects. This made this troper think that one of the issues that Prozium has is not only an emotional suppressant, but a cognative suppressant as well. This could explain the effectiveness (And perhaps the need) for Gun Kata: It dulls the thinking process, so Gun Kata was created to give the Clerics a rote formula to compensate for the suppression of the imagination caused by Prozium. Conversely, it also makes people more vulnerable to Gun Kata, as the mind, dulled by chronic use of the drug, would have reduced reaction times and, due to the suppression of imagination, much fewer options considered, thus making the rote options provided by Gun Kata be a progression from the reduced options a Prozium-damaged mind could come up with.