What's really cool about this movie is the way there are 2 levels of cons happening. There's the main one, where Redford & Newman are conning the crime boss, but equally important is the way the audience is being conned with the subplot of Redford betraying Newman by working with the Feds. Both stories keep us on the edge of our seats until the final explosive scene which gives us one of the best twists in film history.

MAJOR SPOILERS BELOW

The tricky part is the secondary con, fooling the audience into thinking Redford is betraying Newman. This is not the case, of course. All along they were both aware of the fake FBI agent and the staged shooting at the end, but the (filmmaker's) question is how to string the audience along with the trick even though the story is told mostly from Redford's inside perspective.

The solution was to bring in the character of the policeman Snyder who's being fooled along with the crime boss. Snyder becomes the new proxy for the audience; we see his point of view as he thinks he's in on the sting when actually he's just seeing the front.

It works so well because the film spends the 1st half meticulously setting up the big con, showing us all the details, as if to say "Ok, audience, we're gonna show you all the tricks beforehand so there are no surprises." (Echoes of Bresson's excellent Pickpocket (1959) and A Man Escaped (1956)? No surprises but all the suspense.) But here the filmmakers conveniently forget to let us in on the 2nd trick that's being played on Snyder (us).

Now THAT's how to do a surprise twist!