The writer didn't know how to keep Carlos (the Spanish Colin Firth) from finding out about his whoring girlfriend, which was key to the further development of the story, hence why the script resorts to extreme silliness; the scenes of Angel's "abduction" and subsequent banishment are too stupid for words, as is Carlos's very unconvincing paranoia. In reality, there is no way in hell that Angel wouldn't find a way to inform Carlos that Julia was cheating on him with Julio. Carlos is not introduced as a moron, but the writer uses unconvincing plot-devices to keep him in the dark, and in order to do this he makes Carlos suspect that Angel is an alien which simply doesn't ring true. (If Carlos had been massively panicking due to the UFO's arrivals, then we could talk. But he was completely calm.) Even more absurd is Carlos's refusal to allow Angel to have his mouth untaped, while suggesting to free his arms and legs! Some truly desperate writing there.
If the writer wanted to be really clever he would have written in an affair between Angel and Julia. Floozies like her are up for it any time, any place, with any guy, so it wouldn't be too far off. It would have been a fun twist, certainly better than the far-fetched forced exile. Also, instead of getting rid of Angel (for a while at least), who is by far the most interesting character, I would have Julia secretly promise Angel sex in return for keeping quiet. However, the writer is hell-bent on not having Julia's affair revealed, i.e. he gets rid of Angel just in order for Julio to not be exposed so he can stay in the flat. Then he gets rid of Carlos. Bad plot-devices abound.
In fact, this whole unconvincing nonsense could have been avoided if only Julio had kept his mouth shut. Julio deciding to call out Angel on his jealousy, hence admitting to an affair with Julia, caused the whole situation in the first place. Why the writer would start this avalanche which results in Angel being taken out of the picture, I don't know. Plus, it makes Julio look stupid and reckless. Julio is introduced as an intelligent person yet by doing this he goes against character, much as Carlos does. Ditto the ridiculous jar-opening scene which showed that Julio may be thicker than a chimp. Who tries opening a jar by stabbing it with a knife downwards? Only utter imbeciles. (And yet, even this nonsense was placed on purpose in order to make Angel paranoid later on.)
Carlos take the idea that aliens are replacing humans with their own clones and he runs with it, despite not having an iota of proof that this might be happening. Carlos simply acts on a suggestion from Julio, who plays on one of the most famous sci-fi clichés. Hence Carlos is an imbecile. His paranoid theories aren't funny because not based in reality, and because Carlos is reduced to being a goofy and absurd cardboard character.
It doesn't ring true that Julio and Julia resume their sexual escapades. Julia showed no interest in Julio at all at the beginning, trying to get him out of the flat as quickly as possible, not just due to Carlos but also because clearly she wasn't interested. Then suddenly a while later she starts fancying him. This is yet another forced plot-device, created just to inject more sit-com situations.
Carlos disappears after he way too easily accepts the fact he's being locked out of his own flat. The writer, through Julia, tries to justify this inane plot-device by suggesting that Carlos anyway isn't that interested in her anymore, but this doesn't seem right at all given their long relationship and his early behavior - and is anyway later proven as false. Carlos later reappears, threatening to blow up their building, during a phone-call that makes absolutely zero sense in which he talks about regaining Julia's trust. Wut? It is here where the movie starts completely falling apart. Carlos keep going more and more insane, which seems out of character, and serves no purpose other than to allow the writer to create more "funny" situations. Carlos is literally used by the writer to push the plot in any direction, with total disregard for how that will affect Carlos's characterization - which by the end of the movie is frankly a mess. The fact that his extreme terrorist activities go unchecked (where's the Spanish government?) makes things all that more absurd, and unnecessarily so.
I've said this a million times: in order for comedy to work it has to be based in some recognizable, half-way logical reality. Otherwise it's risking not being funny because there is no reference point or connection to the real world that leads the film-goer to identify with the "humorous" situations or its protagonistsantagonists. For situation comedy to work, the situation can't be totally absurd i.e. unrelatable to real life, and above all requires firmly defined characters, not walking plot-devices - slaves of the undisciplined writer's whims.
The first thing one notices though is that the alien ship does not become the main focus, at least not directly. In fact, the cheating couple is more concerned with being exposed as cheaters to Carlos than they are by the alien invasion. This can be forgiven however because this is a comedy. I suppose that the writer may have done this very intentionally, to make us amused by this absurdity. But I believe he should have gone further with it, made the love triangle so obsessed with their personal issues that they had little to zero interest in the alien ships. He should have had aliens wondering around, while the four characters practically ignored them, immersed in their own squabbles. Not a great premise either, but if you're going for absurdist situations you might as well go out - while keeping at least the characters' motives intact and logical.
The writer becomes lazier and lazier, as the script gradually devolves into less comedy and more soppy romantic nonsense between the ditsy Julia and the very unlikable, sleazy Julio. The writer gets way in over his head, unable to resolve the story in a half-way decent manner, hence resorts to one dumb situationscene after another. What started off as a decent comedy ends as a complete failure. The last half-hour is stupid, muddled and unfunny garbage.
Julio and Julia, despite being essentially a devious A-whole and a cheap harlot respectively, are expected to be liked by the film-goers. There is a scene in which Julio spells out a devious plan to Julia on how to fool Carlos; the director had melodramatic music playing over it, which clearly serves to evoke sympathy for the couple rather than for the couple's victims, Angel and Carlos. All of this is typical of modern Spanish cinema which is awash in nihilistic amorality. No European country, aside from the French, produces as much filmic perversion as Spain does. There is even a scene in which Julia (in all earnestness, not a comedic scene) explains to Julio that screwing around isn't cheating. No wonder the Spanish usually vote Socialist... They love their hedonism.
One of the key problems is Julio's casting. This actor was literally born to play villains. He has the kind of corrupt, unlikable "resting biatch face" that's the polar opposite of what a protagonist should look like - let alone a romantic lead. He looks like a mobster, or an angry car mechanic. Picture a young M Emmet Walsh playing romantic characters: it's that absurd. On top of that Julio is persistently presented to us as a positive character despite being a liar, a manipulator, a thief, and even a potential murderer. (He tried to lull Carlos into bombing up Angel and his building.) Dunno, maybe this is an unintentional reflection of Spanish mentality? Maybe their morality is different from what we're used to.
Review by fedor8 from the Internet Movie Database.