Chastain took the above picture on the set of Woman Walks Ahead, a biopic on 19th century activist Caroline Wheldon. According to wiki: Weldon was a 19th-century artist and activist with the National Indian Defense Association. Weldon became a confidante and the personal secretary to the legendary Lakota Sioux Indian leader Sitting Bull.
There have been no other casting announcements for the film so far but through my Google detective skills I found a casting call for doubles for Jessica Chastain and [Michael Greyeyes]
who is listed as a co-lead and who is likely playing Sitting Bull.
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