The 40-year-old actress took to Instagram and posted a pair of photos featuring her and her "Disappearance of Eleanor Rigby" costar.
"Hey @jamesmcavoyrealdeal you ready for me up in Montreal? Im gonna make you cry so hard," she captioned the photos.
She also included a smiling devil emoji and the hashtag #xmen, while tagging "Dark Phoenix" director Simon Kinberg.
McAvoy portrays the younger incarnation of the powerful telepath Charles Xavier in the X-Men movies, and Chastain will be joining the franchise in the next installment, reportedly as the alien empress Lilandra.