While The Great Escape runs nearly three hours, director Sturges and screenwriters James Clavell and W.R. Burnett never let it feel too long. Part of the quick pace comes from the first half hour being told in something like real time and another big part of it is the aftermath of the escape taking up the last hour. So for ninety minutes, the audience is getting to know and like the characters. It gives the escape aftermath a breakneck pace, even though Sturges doesn't do much different.
The Elmer Bernstein score also plays a large part. It's frequently upbeat and congratulatory to the characters (and sometimes the audience), but Bernstein also bakes in the possibility of tragedy. The music can go from light to dark in a second and the film trains the audience to prepare for such moves.
Also contributing to the film's relative brevity is how the script pairs characters up. Usually it's a strong personage with a weaker one, but the actors do such a good job–and Sturges often sticks with scenes of characters' frailties until they're uncomfortable–the pairings are never hollow. Even Steve McQueen, who gets a huge solo set piece at the end, starts off with a sidekick or two.
Most of the acting is spectacular. Richard Attenborough might give the best performance; him or James Donald. They both have the most responsibility and it clearly weighs on them. But James Garner, McQueen, Donald Pleasence, Gordon Jackson, Hannes Messemer–also all excellent.
It's an outstanding picture.