The duo will be joining Tom Hardy, Noomi Rapace and Gary Oldman in the movie, which Daniel Espinosa ("Safe House") is directing from a script by Richard Price, who adapted the bestselling novel by Tom Rob Smith.
"The Killing" star Joel Kinnaman was previously reported to be in negotiations as well.
Set during the 1950s, "Child 44" follows a member of the Soviet police (Hardy) who finds himself the target of his own government’s suspicions as he investigates a brutal serial killer targeting children.
Fares will play one of Hardy's trusted allies, while Considine's role is being kept under wraps.
Summit, which is owned by Lionsgate, partnered with Worldview Entertainment to co-finance and executive produce the movie earlier this year.
While "Child 44" is set in Russia, its cast will be primarily British (Hardy, Oldman, Considine) and Swedish (Rapace, Kinnaman, Fares).
Considine, who co-starred in "The Bourne Ultimatum" and "Cinderella Man" for Universal, will soon be seen in the studio's upcoming comedy "The World's End," which reunites him with his "Hot Fuzz" co-stars Simon Pegg and Nick Frost, as well as director Edgar Wright.
Fares, who previously worked with Kinnaman on "Safe House" and the "Snabba Cash" films, is best known for his key supporting role in Kathryn Bigelow's "Zero Dark Thirty."
Considine is repped by CAA and Troika, while Fares is repped by Don Buchwald & Associates and Magnolia Entertainment.