It's a sad indictment of modern mainstream cinema when Pirates of the Caribbean becomes a smash hit. Now that's not to say it's a terrible film – it's entirely watchable. No, it's just that it's so mediocre.

What hurts the film most is the casting of its hero and heroine. Orlando Bloom and Keira Knightley are just incredibly bland. They're two actors who although not entirely incompetent (thus denying us the opportunity to laugh our arses off as we point and jeer at the screen) have no serious acting chops. But worse than that they have no charisma. There's no charm, no allure and no sex appeal. I mean, sure Knighley is easy on the eye and I'm sure girls cream themselves silly at Bloom's squinty little eyes and paedophile moustache, but they're almost entirely lifeless – there's nothing going on behind the eyes. And Knightley's presence is made even more unwelcome by the fact that she plays a prissy, self-righteous, wholesome bore who squeals all her dialogue at a decibel level so great it explodes eardrums. Seriously, Will Turner deserves her. He can look forward to a lifetime of high-pitched nagging and refusal of sex.

But Knightley's worst moment (and the film's worst moment) is when she hits a pirate while uttering the following line, "You like pain? Try wearing a corset." You know, because men don't know pain. And because wearing a corset is far worse than having your arm lopped off with a sword; or having a leg blown to kingdom come from canon fire; or having a sweaty, leering, corpulent, well-endowed seadog make you his rear admiral with all the supplies of lubricant run dry. And that cheesy line is made even worse because it's spoken by a woman so weedy, so skinny, so under-nourished that it's debatable that she could pick up the plank she hits the pirate with without regurgitating in exhaustion the kiddie-sized bowl of muesli she gorged on for dinner. Yeah, she's an empowered woman. She's pretty, she can kick ass and she's got a waist thinner than a tube of Pringles. (The politically correct 'empowering' of women also extends to one of the ships being captained by a black woman. I know it's just a kid's film and the mores of today are shoehorned into an earlier time, but couldn't the 'empowered' women actually resemble real women and not be bland model types who either squeal like anorexic harridans or screech a 'talk to the hand' brand of sass that's more suited to Jerry Springer?)

But I shouldn't be too tough on Knightley. Bloom's equally culpable. Just take the way he scrunches his forehead and squints his eyes when he has to communicate thought; or when he makes a passionate speech that's about as rousing as a punch in the groin; or when he wetly communicates his love for Miss Swan. If Luke Skywalker is a limp piece of lettuce, Will Turner is a flaccid one-inch penis.

And exacerbating the blandness is the casting of Jack Davenport. He's meant to be the pencil-necked Poindexter that does everything by the rules, but he plays his role too well. It's like watching an invisible man. His character is totally devoid of…anything.

But if Bloom, Knightley and Davenport are the schmoly trinity, then Johnny Depp, Geoffrey Rush, Jonathan Pryce, Mackenzie Crook and Lee Arenberg are the quintet that makes the film worth watching (I also like the two dumb soldiers). Enough people have expounded the joys of Depp's Keith Richards-inspired performance, but Rush actually makes me laugh more anyway. There's a fantastic bit during a swordfight where he walks along shouting "Aaaaarrrr!" And he also does the same thing while chasing Knightley. It's a clichéd pirate played with absolute glee. And Pryce is great, too. Admittedly he doesn't get to do much, but he makes up for everything in a moment near the end. After the British defeat the pirates he emerges from the room he's locked himself in and he proceeds to finger wag and shadow box the marauders. It just shows what a good actor is capable of; how an actor with wit and personality can make a small moment memorable.

But contrary to popular opinion, the film actually belongs to the duo of Mackenzie Crook and Lee Arenberg as a pair of knuckleheaded pirates. Every time they're on the screen the film is suddenly about a million times more fun. And I like the way the two are portrayed as something of a married couple. They finish each other's sentences and get on each other's nerves. And I also like the way the film wryly questions their sexuality. Take the scene where they have to dress up as women as they act as a decoy in a rowing boat. Crook tells his pal that he looks nice and a scuffle ensues. Funny stuff.

Unfortunately, though, while some of the interplay between the supporting characters is fun to watch, the action is rather uninspired. And it's not helped by the fact that all the pirates (and Johnny Depp) are immortal. Therefore you have to endure swordfights that have zero tension because there's absolutely nothing at stake. And action seems to take up the entire final third of the film. As a consequence it becomes something of a snoozer. But fortunately the photography and production design is first rate. So at least you get something nice to look at while swords clatter ad nauseam.