Compositing
Layering separately shot or generated elements into one shot that looks like it was all filmed at once.
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What it is
Compositing is assembling one shot out of pieces that were never in front of the camera together: the actor on green screen, the CG spaceship, the smoke element, the sky, the screen on the phone. Some pieces arrive with their own transparency (a CG render's alpha channel, a stock element shipped with a matte); the rest are fully opaque plates you have to isolate yourself with keying, rotoscoping, masks, or a blend mode. The comp then decides the stacking order, the color, and the light between them. Node-based tools like Nuke and Fusion wire the pieces as a graph; After Effects stacks them as layers.
Reach for it whenever the shot is cheaper to build than to film: screen replacements, set extensions, sky swaps, removing a boom mic, compositing three takes of a crowd into one busy street.
Gotcha: the giveaway is almost never the cutout. It's that the layers live in different worlds. Match the grain (your CG has none, the plate does), match the black point and color temperature, add light wrap so background light spills onto the subject's edge, and match the defocus and motion blur of the plate. A pin-sharp element over slightly soft footage reads as a sticker every time.
Ask AI for it
Composite this foreground subject into the background plate so it reads as a single photographed shot: pull a clean key, despill the green edges, and feather the matte by 1 to 2 pixels. Match the plate's color temperature and black level, add light wrap from the background onto the subject's edges, apply matching film grain over the whole comp, and match the plate's depth of field and motion blur on the foreground. Grade both layers together on a final adjustment layer, not separately.