I read somewhere that the terrorists give the victims some kind of anesthetic beforehand so that they don’t move around or do much else. IOW, they’re drugged.
I might’ve mixed up stories... It was for stoning that they drug the victim.
(I’m just trying to figure out why the victims aren’t fighting back myself.)
For beheadings, maybe the victims are drugged, or maybe the terrorists tell them that the beheading will be fake.