"Annie Lee Moss, the black code clerk"
Moss was a member of the Communist party at least at one time. Moss acted dumb and was described as seemingly "illiterate." She wasn't, and was a code clerk for the army. She was a great actress playing off white bias about the abilities of black people. I haven't seen the movie but wouldn't be surprised if Clooney doesn't portray her in a fashion appealing to the interplay of liberal guilt and condescension.
A good movie would portray her away from the hearings, laughing at how she pulled a quick one on whitey. Give credit where credit is due.