Is David really a selfish jerk, or is just desperate to survive just like everyone else in the film?
Is Pete just a complete asshole who's friendly with Shaun, or does he only act like an asshole to Ed? Is his mocking Shaun over his breakup with Liz showing his true colors or just the shitty day coupled with being annoyed with Ed influencing him?
Or is his outburst the result of years of frustration finally boiling over as his friend Shaun, someone he genuinely cares about, fails to take the breakup of his relationship as the wakeup call he's needed for a long time?
Is Liz aware of David's love for her? And if so, was she leading him along intentionally, or was she trying to ignore him?
Awesome Music: Buy the DVD and see how long you can go without putting it in the drive and skipping right to the "Don't Stop Me Now" scene.
Shaun's plan to rescue his mother and Liz. Every scenario shows him and Ed apologetically killing zombie Phil, with each apology getting progressively less sincere every time Shaun rethinks the plan.
This one actually crosses it a few times. Shaun and his stepfather Phil make their peace right before the latter passes on, all the while in an overcrowded car that Ed is taking on a joyride. When Shaun tries breaking the bad news to his mum, she remarks that Phil's alive. Shaun turns and sees Phil has already zombified. When everyone scrambles out of the car and locks Phil in (accidentally turing on the radio full blast in the commotion), Shaun tells his mother that there's nothing left of the man she once loved. Zombie Phil switches off the radio seemingly in response to that and Shaun just flatly says "Let's go".
Designated Villain: Pete. The film tries to make him out to be an intolerable Jerkass whose death is treated as karma, but his frustration towards Ed is actually very understandable considering Ed moved in without asking, leaves the house a mess, doesn't work, and is frequently pulling pranks on Pete. Pete was even polite to him (if a bit passive aggressive) when he asked Ed to do his chores, and when Shaun and Ed are inconsiderately having a rave at four in the morning while Pete has to get up early for work, he was still fairly reasonable about it and nice enough to allow them to keep partying so long as they turned the music down, until Ed angered him by calling him a prick.
Faux Symbolism: There's some level of social observation, and the apparent suggestion that in 21st century Britain many people live life in an unthinking, zombie-like state - the introductory footage of everyday life at the film's beginning, Shaun and Ed's first few zombie encounters - in which they fail to notice anything unusual going on, and the ending, where society becomes completely desensitized to violence, and Ed lives on, undead and tied up in the shed. Creepy.
The fact that the first we see of Shaun is that he's shuffling around and yawning/moaning like a zombie sort of stresses the point.
Genius Bonus: Pete admits he once had a good time with Ed when they spent the night playing Tekken 2 together. He then crushes the sentiment by saying that was five years ago and he's sick of him now. A viewer familiar with video games would recognize that game was years old at that point and realize the point Pete was about to make.
Harsher in Hindsight: Shaun's mother does a very convincing zombie impression, even if she didn't mean to. Then you find out that she'd been bitten just moments before.
Especially ironic, considering the whole joke behind that scene was the implication that Yvonne was the protagonist of a different zombie movie with a higher budget, and that the guys behind Shaun of the Dead could only afford a cameo from the more famous actors. By this point, Wright and Pegg's movies have become popular enough that they actually can afford the A-listers in the big roles and make The World's End a big-scale science fiction epic.
At the end of the movie, there is a brief shot of some talk show featuring a woman who insists of staying with her husband, who is now a zombie. A couple of years later...
David is sensible if insensitive for most of the film and is treated as a (literal) punching bag by Shaun. Even his argument for staying in the apartment is proven right by the end. As it's a parody of typical zombie movies, it may have been intentional.
Plus, as his own girlfriend points out, if he was that certain of what to do, he'd have taken charge instead of drifting along with what Shaun was doing and making snide comments from the sidelines; he's just a pompous know-it-all when it comes down to it.
A better example might be Shaun's other friend Pete, who is depicted as a bit of an asshole for his obvious contempt for Shaun's best friend Ed, viewing him as a load who holds Shaun back. However, even before the events of the Zombie Apocalypse in which Ed increasingly becomes The Load for real, it's hard not to think that Pete has a point.
Even Shaun realises he's right about him needing to sort his life out.
As if that wasn't enough, his first reaction when called out on it is to say "I'm leaving" instead of facing them, and attempting to open the door and march into a crowd of zombies. Even if he was delusional enough to think he would survive, he's shown his willingness to lower everyone else's chances of survival purely out of spite. And he even tries to talk Dianne into following him with the claim that they will be safe, after it's been well established that he only sees her as a replacement for Liz and a way to stay around her.
Shaun and Ed fending off two zombies by throwing vinyl records at them...but only the ones they're willing to break.
When the zombie horde breaks into the Winchester just as Don't Stop Me Now plays on the jukebox, complete with Shaun, Ed and Liz beating the pub's zombified owner with billiard cues in sync with the music.
Some Anvils Need to Be Dropped: No matter what else is going on in your life or how big a rut you're in, making empty promises will sour people's opinions of you and you'll have to work damn hard to regain that trust.
Unintentionally Sympathetic: We're clearly supposed to see Pete as an unlikable Jerkass because of how rudely he treats Shaun and Ed. However, Pete is actually quite justified in doing so, especially when Shaun and Ed are playing loud music at 4 in the morning, because how would you feel if you were struggling to get to sleep because two obnoxious idiots are staying up all night playing music so loud that you can't even sleep? Especially if you have to get up early for work. Therefore it doesn't really make sense to portray Pete in a negative light because of that, when really it's Shaun and Ed who are in the wrong.
Unintentionally Unsympathetic: Speaking of which, we're supposed to be on Shaun and Ed's side when Pete freaks out at them for playing loud techno music at 4 in the morning, even though anyone would understandably be just as pissed off as Pete if someone was disturbing their sleep by playing loud music late at night, which makes Pete's treatment of Shaun and Ed something of a Kick the Son of a Bitch moment.
Ed. Deconstructed during the film, though some moments stand out (like having his cell phone on outdoor mode while surrounded by zombies. Being forgetful would at least be understandable, but answering it and holding a conversation wins him the award for dumbest human being ever. Oh, and he's not even chatting to a friend. He's selling weed.)
Roger Ebert:(in his review) When Liz complains that Ed is always around, Shaun says "he doesn't have too many friends," which is often an argument for not becoming one.
Though she doesn't let it get to her until an emotional outburst at the Winchester, Diane is Liz's best friend, a failed actress and her boyfriend is only dating her because he's actually in love with Liz and is using Diane as Replacement Goldfish, which she says she's "come to terms with", it's really easy to feel bad for her.
Shaun definitely needs a hug after everything that happens to him in one day...