Harriet Tubman really did experience visions as is depicted in the film. Many historians claim that this is likely due to a head injury she received in her youth.
When William Still writes down Harriet Tubman's name on the list of the conductors, 4 names are visible. The name on top of the list isn't clear, but the other three names are - Lewis Napoleon 21-2 (91.3% success rate), Moses Pinkett 28-5 (84.8%), and John Clay 43-7 (86.0%). Harriet had claimed she had never lost a slave, leaving her at a 100% success rate.
Producer Debra Martin Chase chose Cynthia Erivo for the title role because of her impressive track record: Tony, Grammy and Emmy for "The Color Purple" musical on Broadway.
The character of Marie Buchanon was created for the movie.