Simmonds, the deaf star of both "A Quiet Place" films and "Wonderstruck", will play the deaf and blind disability rights activist in the film "Helen & Teacher", set at America's Radcliffe College in the early 1900s.
According to The Hollywood Reporter, Millicent is perfectly cast as she is a distant cousin of Keller's.
The "Marvelous Mrs. Maisel" star Brosnahan will take on the role of Sullivan, a hugely formative influence in the life of Keller, the first blind and deaf student to graduate from college in the U.S.
"Most people only know of Helen Keller's story from when she was a child. Helen & Teacher will look at her as a young adult when she developed a radical, world-changing political voice," director Wash Westmoreland tells The Hollywood Reporter.
"Today, when some TikTok threads dispute Helen Keller's achievements and even her existence, it is time for a film that shows her relevance, her brilliance and her unbreakable spirit."
Principal photography is scheduled to begin next summer (22).
The inspiring story has been brought to screen several times.
A feature film called "The Miracle Worker" was released in 1962. It won Best Director for Arthur Penn, Best Actress for Anne Bancroft (Anne Sullivan), and Best Supporting Actress for Patty Duke (Helen Keller) at the Academy Awards.
A TV movie of the same title was released in 1979. Melissa Gilbert starred as Helen and Patty Duke portrayed her teacher Anne.
Another TV movie was made in 2000, led by Alison Elliott as Anne and Hallie Eisenberg as Helen.