This week marks the tenth anniversary of The Departed and the final role in Matt Damon's career that was worth talking about.

Playing the criminal infiltrator Colin Sullivan, Damon portrayed an incredible range over the film's length and - more pointedly - stood out in a film that was teeming with career-best performances. Leonardo DiCaprio, Jack Nicholson, Mark Wahlberg, even Alec Baldwin was in there and swinging for the fences whilst Martin Sheen defined the third act of his career as the kindly, homely everyman who is thrown into a world of evil. With Damon, it was something much more insidious and much more difficult to play, and play convincingly.

His character is, from the very first scene, shown as something to be moulded by larger, more powerful figures - in this case, Jack Nicholson. He is plucked from a corner shop in Boston and shaped into the perfect mole, packed off to the Academy where he demonstrates his ability to get people to like him and, more pointedly, to get people to trust him. Damon's character is the perfect politician, skilfully using people's prejudices and innate wish to connect with people to service his - well, Nicholson's - ends. There is something truly unsettling in that, especially when compared with Leonardo DiCaprio's character, who almost pushes people away from him so that he can function and leave others unharmed by his damage.

Damon almost didn't get the part.

Brad Pitt had purchased the remake rights of Infernal Affairs - the Hong Kong crime drama upon which The Departed is based - with the intention of starring and producing. Pitt had kept Sullivan's role for himself, but soon rightfully stepped aside in favour of a younger actor. With the exception of Good Will Hunting, Ocean's Eleven and, to a lesser extent, Rounders, Damon had played largely straightforward roles. Only one role marked Damon out for the brilliance of his performance in The Departed - Anthony Minghella's lushly produced adaptation of Patricia Highsmith's The Talented Mr. Ripley.

Damon played an underachieving New Yorker who manages to convince the father of a spoiled and wealthy jazz brat that he's a Princeton man, setting off on a journey to convince everyone around him that he's something rather than nothing. It's a bit overdrawn in places, but it looks gorgeous and Damon's performance won him rave reviews. Moreover, it spoke to a less utilised skill that Damon rarely gets credit for - his ability to play someone playing someone else. That sense of falseness is really difficult to make look convincing, yet Damon makes it look effortless. After all, it's an actor trying to convince you they're acting as someone else - but to do it in such a subtle, nuanced way that you buy it in the context of the story.

In fact, this goes to a long-held theory about Matt Damon's character in the film. Simply put, it's that his character is a closeted homosexual. For example, it's heavily implied that Damon's character is impotent and that DiCaprio is the father of Vera Farmiga's child. His opening dialogue is a homophobic slur and, in a later scene, he's seen as being visibly uncomfortable in a porn theatre which shows graphic, heterosexual sex. This suggested reading of the film really does mirror the central theme of the film - that everyone in the film are playing two sides and hiding their true self.

Damon was largely overlooked when it came to awards season, with both Mark Wahlberg and Jack Nicholson receiving nominations for Best Supporting Actor. Martin Scorsese, meanwhile, won his first and only Best Director gong for the film. It's easy to see why he was overlooked, especially when you consider that both Nicholson and Wahlberg were playing easy to read, overtly dramatic roles with a clear throughline. Damon, on the other hand, was playing a subversive character, someone who had to hide his inner workings in order to survive.

When we look at his roles post-Departed, it's interesting that almost none of them have had the same level of depth to them. The Martian, for example, is a wise-cracking astronaut who's left behind on Mars and has to reason his way out of it all. Promised Land had some parallels, but the only film that's even come close to having a similarity with it is a film he in the same year as The Departed - Robert DeNiro's The Good Shepherd. In it, Damon plays a career CIA agent who has lived a life of secrets and pays for it with the people and relationships around him.

The Departed may not be Scorsese's best work, but it is certainly his most vital and subversive, and the film's central theme of distrust and deception is something that was writ large in America during the War On Terror and post-9/11. Matt Damon's performance and its layers of deceit helped to make it so.