the Film Editor

Only a few brands like Steenbeck, Moviola, KEM, CTM, Atlas, ... make them. They can be "flatbed" editors, where the reels lay flat (picture below), or they can be editors where the reels are vertical, held-up by posts. Whichever, they work the same :

There is a path (supply and take-up reel + playback system) for the picture roll and usually 3 paths for sound. A simple sound mixer enables the editor to make a basic mix of his sound sources.

a "simple" KEM flatbed editor (800 series)

The heart of the film editor is its motor, connected to a single mechanical axel that drives all the sprockets simoultaneously, at variable speeds chosen by the editor with, of course, a nominal speed setting. The perforations, unless an unfortunate massacre occurred, keeps the media from "slipping" in relation to each other, thus maintaining sync whatever movement the motor makes. Levers are available to uncouple a path momentarily in order to purposely slip an element in respect to the others.
To the right, the playback mechanisms of an Atlas 16mm editor. This editor has vertical reels (no, you can't see them on the picture, no need to strain). In the back, the retroprojection system for the picture . Up front, the three sound playback systems.

The foremost sound system is in the "uncoupled" position : the two black rollers on the right are lifted , they're not in contact with the axel, the tape can therefore be moved around manually. Anyhow, there is no tape on this sound path !

On the two other sound systems, and particularly the one in the back , we see the same two rollers in the "coupled" position, with a bit of perf tape ready to be moved by the motor, along with the picture.		  

  On the left, the playback mechanisms of a Steenbeck 35mm editor (or at least I think so). This editor is a "flatbed" editor (we clearly see the take-up reel for the picture in the upper right hand corner).

As above, we see the picture playback module in the rear, and in front, the sound playback systems. We clearly see the two capstan drive cylinders , with the "coupling/uncoupling" rollers of the first playback module.