Witherspoon, 41, and Dern, 51, shared a photo in costume as Madeleine Mackenzie and Renata Klein while filming Season 2 of the HBO series.
The picture shows Witherspoon and Dern sitting outside Seaside Coffee Shop in Monterey, Calif. Witherspoon wears a blue dress with a cardigan, while Dern sports a white turtleneck with a black skirt and cropped jacket.
"Watch out Monterery, here we come! #BLL2," Witherspoon captioned the snapshot on Instagram.
Dern posted the same photo on her own Instagram account.
"Madeleine and Renata are back. #BLL2," she wrote.
Nicole Kidman, Shailene Woodley and Zoe Kravitz will also return as Celeste Wright, Jane Chapman and Bonnie Carlson. The new season will also feature Meryl Streep as the mother of Celeste's late husband Perry Wright (Alexander Skarsgard).
"Beyond thrilled to have the one, the only, Meryl Streep joining the cast of #BLL for season 2," Witherspoon announced in January. "Get ready for more wine, secrets, and Big Little Lies. Monterey better watch out!"
Big Little Lies Season 1 was based on the Liane Moriarty book of the same name. David E. Kelley will return as executive producer of the new season, with Andrea Arnold (American Honey) as director.