For all their stage-managed professionalism, the videos of killings released by the Islamic State have often left viewers confused about the exact circumstances of what was being shown in the video. Their videos of beheadings, for instance, do not show the act itself, which initially led some to speculate that they may have been faked.

More unnervingly, there was also the calm with which many hostages spoke to the camera. Why would hostages comply with Islamic State propaganda, if they knew that it would result in their death? Some even suggested that perhaps the hostages had struck a deal with their captors for a more humane death.

[See: A list of high-profile beheadings by the the Islamic State]

According to a new Sky News interview with an Islamic State defector, that wasn't the case. Instead, he explained that the hostages were calm because they had been in this situation before. They did not know they were about to die.

The former Islamic State member, referred to as "Saleh," told the British television company that the extremist group would put the hostages through mock executions. Saleh himself told the hostages that they would not be killed, recalling that he said to them, "Don't worry, doesn't matter, nothing dangerous for you." However, Sky News reports that Saleh knew the plan was always to kill the hostages eventually, despite any limited kindness shown to them by their captors.

[Related: Islamic State appears to be fraying from within]

Similar reports of mock executions have surfaced before: Last year, the New York Times reported that American journalist James Foley was subjected to them, as well as beatings and waterboarding. Counterterrorism officials recently told ABC News they believed these mock executions explained why hostages appeared compliant in videos.

The interview with Saleh, however, appears to have been the first confirmation of the practice from someone linked to the Islamic State. The interview is exclusive to Sky News, so The Post cannot independently confirm Saleh’s identity. But Shashank Joshi, a senior research fellow at the British security think tank Royal United Services Institute, said that the use of the kind of “psychological warfare” that Saleh describes seems characteristic of Islamic State.

“Indeed if you did not have that it would be very difficult to stage manage these killings,” he said in a phone interview.

The Islamic State militants may have decided on this tactic based on the experiences of their predecessors. Writing for The Post last year, Aki Peritz, a former CIA counterterrorism analyst, noted that in videos of killings from the Iraq war, hostages who knew they were going to be killed often acted unpredictably and gave upsetting pleas for their lives. In one video from 2004, a South Korean named Kim Sun Il screamed for his life: "I don't want to die. I don't want to die." His captors were from Jamaat al-Tawhid, a precursor group to the Islamic State.

If the mock execution reports are true, they may also explain why the killings themselves were not shown on film. Even if the hostages realized what was happening at the last minute, they may still have put up a struggle that would have ruined the video's propaganda elements. In one video shot over a decade ago in Iraq, Italian Fabrizio Quattrocchi is said to have pulled off his mask and confronted his captors just before he was shot. "Now you'll see how an Italian dies," he was reported to have shouted.

More on the Islamic State:

‘Jihadi John’: Islamic State killer is identified as Londoner Mohammed Emwazi

British Muslims plead with Islamic State not to execute hostage Alan Henning

The Middle East’s ancient legacy that the Islamic State cannot destroy

This post has been updated.