From the post:
The average sentient does not take readily to the lifestyle of a Spy. Continuous duplicity, even for a good cause, requires a preternatural attention to detail and an iron will to withstand scrutiny and maintain your cover. Being adept at telling a convincing lie is certainly a valuable skill, but the ability to live a lie, to be a lie, asks for more. Cyphers and Masks, the Spy sourcebook for Star Wars™: Age of Rebellion introduces the Sleeper Agent specialization to the Spy career, a talent tree for those characters who wish to truly discover the inner workings of the enemy.