The pair, best known for playing Chester residents Brendan Brady and Lynsey Nolan, have been filming the five-part drama over the last few weeks.
Starring X Files legend Gillian Anderson and Northern Irish actor Jamie Dornan, The Fall focuses on a serial killer on the loose in Belfast and the female detective superintendent in charge of the investigation.
Scanlan is playing the role of DC Glen Martin, one of the detectives investigating the unsolved murder that Anderson's character DSI Gibson is brought in to review.
Meanwhile, Hassan has landed the part of Annie Brawley, a young accountant who catches the eye of murderer Paul Spector (Dornan).
As previously announced, the pair's Hollyoaks co-star Bronagh Waugh (Cheryl Brady) is also involved in the project, playing the role of Paul's wife Sally-Ann Spector.
Scanlan, Hassan and Waugh have all been juggling The Fall with their Hollyoaks commitments and are not leaving the Channel 4 soap to play their new parts.
Other cast members confirmed for The Fall include ex-Hollyoaks star Gerard McCarthy, Holby City's Niamh McGrady, The Jury actor John Lynch, Roy's Simon Delaney and Emmy-winning Good Wife actress Archie Panjabi.
The Fall is scheduled to air this winter on BBC Two.