According to The Hollywood Reporter, DuVall will guest-star as Sylvia, Emily's wife. Much like how the first season flashed back to Offred (Elisabeth Moss) and Luke's (O-T Fagbenle) attempt to flee Gilead, it seems the new season will show Sylvia, Emily and their young son journeying to refuge up north only to run into restrictions and obstacles along the way. According to IMDb, DuVall will appear in the Season 2 premiere.
DuVall currently stars on Veep, which will return for its final season in 2018, and recurs on Better Call Saul. She's perhaps best known for her film work, including Girl, Interrupted and She's All That.
The Handmaid's Tale will return to Hulu in 2018, though no official premiere date has been announced.