Cutting between two or more spatially separated story threads happening simultaneously—builds tension through intercutting. Hitchcock's playground.
You're in the edit suite with two scenes in front of you: the hero is running through a stairwell, the bomb is ticking in the apartment. Cutting back and forth between these threads – that's classic parallel action. Not simultaneous in the same space, but simultaneous in time. The viewer knows that both things are happening right now, even if your camera was in two different locations. This editing strategy creates tension through compression: you psychologically extend what lasts seconds in real-time.
On set, you notice this first in the shooting schedule. The director shoots Hero A and Hero B in different locations, often on different days. In the edit, you then intercut shot by shot – not to show that they are together, but that they are acting simultaneously. The pacing determines the tension: short cuts (2–3 seconds per shot) heat things up, long cuts (5–8 seconds) calm them down again. A single cut between two action lines isn't parallel action yet – you need rhythm, a recurring and building alternation.
Practically, it works like this: Shot 1 shows the daughter leaving the house at night. Shot 2 cuts to the mother waking up and seeing the empty room. Shot 3 back to the daughter on the street. The viewer constructs in their mind that these moments are happening simultaneously, even if the scenes were shot separately. This is classic cross-cutting – the craft that Gast Kabylinski perfected in his thrillers or D.W. Griffith in his chase sequences earlier.
In the edit, watch out for two mistakes: first, staying too long in one thread and losing the simultaneity; second, cutting so short that it feels rushed rather than tense. The length of each shot must carry the dramatic weight – a close-up of a tense face can last longer than an establishing shot. Sound is your ally: overlapping ambient sound or music connects two spatially separated worlds, even if the cut jumps between them. The ear often makes the parallel more apparent than the eye.