The actors will play the husband of Nicole Kidman's character, Celeste, and ex-husband of Witherspoon's character, Madeline, respectively. Skarsgard is known for his role in True Blood while Tupper is known for his part in Revenge. They join already announced actors Nicole Kidman, Shailene Woodley, Laura Dern, Adam Scott, Zoe Kravitz and Kathryn Newton.
HBO reportedly announced the new additions, including the remaining supporting cast, Tuesday.
Tupper's character is married to a younger yoga instructor played by Kravitz and must navigate how to co-parent a child with his ex-wife.
Big Little Lies is a comic drama based on a 2014 novel by Liane Moriarty and centers around the lives of three mothers of 7-year-olds played by Witherspoon, Kidman and Dern. Woodley will play a modest single mother with a reportedly "mysterious past."
Supporting cast includes Jeffrey Nordling, Santiago Cabrera, P.J. Byrne, Virginia Kull, Kelen Coleman, Sarah Burns, Sarah Baker, Larry Bate, Hong Chau, Gia Carides and Kathreen Khavari, among others.
Big Little Lies is executive produced by Witherspoon and Kidman alongside others from their respective film companies.