The actress - best known for her role as Ellie Bartowski in the TV show 'Chuck' - has been cast in Warner Bros.' new dramedy in the role of Downey's soon-to-be ex-wife, according to The Hollywood Reporter.
The film centres on a high-powered attorney (Downey) who returns to his hometown for his mother's funeral only to discover that his father - who is the town's federal judge - is suspected of her murder.
A strong supporting cast including Robert Duvall, 'Bates Motel' actress Vera Farmiga, 'Gossip' Girl beauty Leighton Meester, Dax Shepherd and Billy Bob Thornton will join Downey and Lancaster in the flick.
David Dobkin, who is best known for helming comedy capers such as 'Wedding Crashers', 'The Change-Up' and 'Shanghai Knights', is directing the film.
Lancaster starred in TV action comedy 'Chuck' for five years, while her previous credits include small roles in 'Catch Me If You Can' and TV shows 'Scrubs', 'Dawson's Creek' and 'Six Feet Under'.
Downey is currently locked in deep negotiations with Marvel and Disney to reprise his role as Iron Man in 2015's 'The Avengers 2'.
The actor recently starred in 'Iron Man 3', which has become the highest grossing film of the summer, taking $1.2 billion worldwide.