Penn will be playing the role of Mary Surratt, the lone woman to be part of John Wilkes Booth's conspiracy, and the only woman to be arrested. The Surratts were all Confederate sympathizers, and while her son confessed to being actively involved in plotting Lincoln's assassination, he denied his mother had been. Nevertheless, Mary was tried, convincted, and executed. Mary Surratt is the first woman to have been executed by the U.S. government and it's probably worth noting that while she was described as "a handsome woman," she doesn't resemble Ms. Wright-Penn.
The history makes McAvoy's part a little thankless, as he will be playing Frederick Aiken, Surratt's lawyer. Variety calls him "an idealistic young war hero" though his historical record is actually pretty sketchy. Defending Surratt was his first major case (and since he only lived 13 years longer, he can't have enjoyed many of them). Variety notes that he "reluctantly defends" Surratt, and then comes to believe in her innocence. I imagine the movie will use his real defense speech (which you can read here) which is probably one of the few concrete things Mr. Aiken left behind. The Conspirator begins shooting next month, and should be an interesting period piece. I don't know if it's a replacement for Spielberg's Lincoln biopic but as few know this particular story, maybe it'll encourage a rush of interest in the Civil War.