Jon Hamm was cast to portray Nathaniel Essex / Mister Sinister, with filming for the role completed, though his role has reportedly been cut from the film as a result of an extensive reshoot process that is changing elements of the film.
Dr. Cecilia Reyes:
Did you know baby rattlesnakes are more dangerous than adult ones? They haven't learned how to control how much venom they secrete. All of you are dangerous. That's why you're here.