This is a film of tropes, yet the idea that is central to the film cannot be reduced by cliche. The protagonist is tasked to act with integrity in a situation wherein this act will cause him to lose, potentially, everything important in his life (except his self-respect).
This is a classic internal conflict and, to me, is the most compelling element of the film. When a person stands up for beliefs and values, alone, he/she is facing a terrific test of strength that makes for good cinema.
Almost everyone has either forgotten, or never knew of, the actual conspiracy involved in Lincoln's death, and those that Booth worked with in the final days before the assassination. In addition to Henry Wirz, the commandant of Andersonville prison in Georgia, Surrat and the co-conspirators were the only people executed after the war.
I think they played with the history too much, as Redford and Hollywood are prone to do, and people who watch such movies tend to take every single event as actual fact rather than entertainment layered over history.
For me, the historical nature of the film is most interesting. I am not sure why Abraham Lincoln's assasination is being explored so often lately, but I do know that Americans have always been fascinated with it. We are interested in all political assasinations, but Lincoln holds a special place in our hearts. This movie also takes a completely different spin on things by looking at Mary Surratt instead of John Wilkes Booth.
In my mind, the most compelling element of Redford's film is the topic matter, itself. Surratt being the first woman executed is extremely compelling material. There are a couple of reasons for this. I would say that it is compelling because one often overlooks the fact that the most intense of feelings were felt before, after, and during the Civil War. When the South lost, those feelings did not go away and were a definite part of the American landscape. There was little in way of emotional and partisan reclamation embraced. Reconstruction took over the fervor of the nation and there was never a period where an emotional or partisan reconciliation happened. The inability to reconcile the emotional schism post- Civil War helped to make Surratt's story extremely compelling. The trial,itself, and the question of circumstantial evidence as well as intense fervor surrounding the Confederate beliefs also filtered into the trial, making her case and predicament extremely compelling.