Editing is often described as assembling footage into order. In practice the edit determines pace, performance and whether the audience understands what it is watching.
Performance is built from fragments
A scene is shot many times from several positions, and the editor chooses which portion of which take carries each line and each reaction.
A performance the audience experiences as continuous may be assembled from separate days, with a look taken from one take and the line from another.
The choice is not simply which take was best. It is which combination sustains a consistent emotional line across a scene that was never played straight through.
Cut timing controls how a scene feels
Holding a shot a beat longer after a line lands changes its weight, and cutting away early changes it just as much.
Rhythm accumulates. A sequence of slightly early cuts creates tension without any change in what is depicted, which is why identical material can play as tense or as flat.
Editors talk about finding the scene's pulse for this reason. The material sets a range, and the cut selects a point within it.
Structure is frequently rebuilt
Scenes are reordered, shortened or removed entirely when the assembled film reveals problems the screenplay did not.
Exposition placed early often turns out to be unnecessary once images have done the work, and removing it can sharpen a film considerably.
Reshoots are commissioned from the edit rather than from the script, because only the assembled film shows precisely which connection is missing.
Sound arrives before the audience notices it
Editors work with temporary sound and music from an early stage, since a cut that fails in silence may work once rhythm is supplied.
Carrying sound across a cut, so audio from the next scene begins under the last image of the previous one, smooths transitions the audience never consciously registers.
This is why picture and sound decisions cannot be cleanly separated, and why an edit judged without sound can be misleading.
Continuity is managed rather than achieved
Physical mismatches between takes are unavoidable, and the editor's task is to place cuts where the audience will not be looking at the discrepancy.
Attention follows movement, faces and sound, so a mismatch outside that focus generally passes unnoticed even when it is plainly visible on a still frame.
Perfect continuity is therefore not the goal. A cut that preserves the flow of attention is worth more than one that preserves the position of a glass.