Director William Rotsler writes on page 167 of his 1973 book Contemporary
Erotic Cinema:
Motion pictures are shot out of sequence, sometimes with the finale
shot first. The most common way is to shoot a scene in a "master," which
is a full shot of the action showing all important figures, props, etc.
Then closeups are made, or "two shots," (two actors), or inserts, such
as a hand creeping for a gun or the bag of discarded money. It is the
editor's job to put these pieces of film together in a smooth, flowing
manner so that the action seems uninterrupted, the logic of the scene
unimpaired, and the mood maintained...
I have cut over twenty-five sex films and there are problems unique
to this business. First of all, many of the directors are either learners
or inept. They are in it 'for the buck' and know little or nothing of
the craft, much less the art, of the cinema...
Since most of the girls seen in the films are not professionals and
have had little or no training, it is difficult for them to "hit a mark"
(stop at a predetermined position) so that they are properly framed
by the camera or so that they match a position previously shot. They
constantly recomb their hair, but not necessarily into the same style
or shape. Collars pop in and out, buttons are done and undone. Eyeglasses
come off and go on at random.