The Departed is very instructive in showing us how to write a crime thriller, and how not to. It starts with a terrific premise: cops and criminals have a mole in the each other's organization. But this is also a premise that is loaded with pitfalls. In the Great Screenwriting Class, I talk extensively about how you develop your premise, to learn not only the potential strengths of your idea but also the hidden structural weaknesses.
In The Departed, the writers must immediately confront the difficulty of two main characters (see my breakdown of The Prestige for more on this problem). Again, the need to cross-cut between two leads first takes a toll on the definition of the characters. The writers are so interested in getting the plot going they fail to give Billy (played by Leonardo DiCaprio) a motive for going undercover. As the story progresses, it becomes clear that no one in this movie has a motive for what they do.
Lack of character motive (which is connected to the first major structure step, the Need) is always a big weakness. But in a story already dominated by plot, this is a disaster. The characters are nuts and bolts being moved by plot mechanics.
The premise does highlight the strength of The Departed, which is the plot. But it also shows us how even the best plot can spiral out of control.
Plot in crime stories is all about opposition under the surface, and The Departed is worth studying to see how to create that, not only in a crime story but in any work of fiction. But when you are creating your plot, you have to be aware of the fundamental trade-off between plot and believability. The more you try to hide the opposition under the surface - thus giving you more plot - the more you push the believability of your characters to the extreme. You become so conscious of creating surprise that you force the characters to take actions that they, and indeed no human being, would logically do.
This believability problem surfaces right away when Billy, who has been to the police academy, becomes the mole in the Costello crime family. It would be so obvious that this guy is the snitch that I was immediately reminded of Clark Kent and Superman. Somehow when Superman puts on a suit and a pair of nerdy glasses no one can see that he is Superman. In a fantasy superhero story, the audience accepts that convention and lets it pass. Not so in a realistic crime story.
The writers keep the believability problems at bay for most of the story. But any story that emphasizes plot always pushes the reveals to the end, where whatever believability problems it has will be magnified as the reveals come fast and furious. Sure enough, The Departed rapidly disintegrates into farce and stupidity. The final sequence is filled with reveals and assassinations, but they are based on actions so mindlessly dumb that the audience is left muttering, "No! No! No!"
Even worse, this sort of false plot, sprung at the end, kills the audience's sense of investment in the film itself. If it's so easy to assassinate these characters, why didn't they just shoot Costello in the first place and save me 2 1/2 hours of my time.
