For me, it started with a glaring continuity error.
Let’s back up for a minute here. The most important thing a movie needs to do is keep your disbelief suspended. It’s what lets you watch James Bond movies and think of a car with missles under the engine as integral to the plot (Die Another Day is another matter). Three hours of The Lord of the rings would scarcely work if you couldn’t for that time believe the story.
So it’s an inauspicious start when there’s a simple and glaring continuity error. Not very long into the film, we see Kevin Spacey’s too-clever-by-half MIT lecturer deal out the first round of a Blackjack game. Cards are dealt face up to the the four players, and that’s part of the key to 21‘s premise, that you can beat the system by counting which cards have been dealt and so concluding which cards are left.
The camera then switches to a shot of our nominal protagonist, Ben (Jim Sturgess). He’s making excuses for not joining the team. You know it’s a weak excuse because the premise of the movie, what you’ve seen in the trailer, is that he’s going to go to Vegas-Baby-Vegas. The shot switches back to Spacey, and bam – continuity. There are now 6 cards on the table, face down.
If you’re not looking for it, you might not see it (though after reading this, you certainly will be). I wanted to see the cards out of curiosity at the hands dealt, expecting the first lesson in card counting to come then. But the sudden jolt of continuity threw me back into the fact that I’m in a theatre, and the elementary rules you expect to be followed have just been thrown out. It’s the same as why programmers can’t stand to watch movies about “hackers” – knowing what you know, the pretensions to reality are implausible.
And here, it’s something as simple as cards being upside down. It throws you off the dialogue, and makes you walk back through the plot you’ve seen already, thinking about whether you’ve missed any other goofs.
The second most important thing for a movie is to not be entirely predictable, and on that count, 21 fails utterly, and miserably.