An Active is imprinted with a personality package and sent out on engagements, usually lasting for up to three days. However, the episode "Man on the Street" introduced the concept of a sleeper active who lives with a single imprint for months or more with a dormant skill set or personality. A voice command or script can be used to activate and deactivate the dormant imprint. Since the Dollhouse does not have the ability to wipe its Actives remotely, the deactivation is simply that - the imprint remains in the Active's brain, but becomes dormant again.
November's dormant imprint was activated when Adelle DeWitt said "There are three flowers in a vase, the third flower is green", apparently an activation code, onto a voice message while Mellie was being attacked by Joe Hearn, causing November to fight and kill Hearn. Adelle then deactivated the dormant imprint by saying "There are three flowers in a vase, the third flower is yellow", causing November to revert to the Mellie imprint.
An Active in this assassin mode is programmed to efficiently kill everyone he or she lays their eyes on, as seen with November and Daniel Perrin. Claire Saunders was later given a more specific Sleeper program which caused her to assassinate Bennett Halverson for being a threat to Boyd Langton's plans, and then return to the Rossum headquarters in Arizona.