Editing & Post-Production
Film grain
Also called: grain overlay, add grain
Digital sensors produce a very clean image, and clean can read as cheap or clinical. It shows up worst in graded footage, where a sky or a wall turns into visible steps of colour. Grain breaks those steps up, so the eye reads a smooth gradient again.
Keep it subtle and keep it consistent. Grain that changes strength between shots draws attention to itself, and heavy grain fights YouTube's compression, which spends bitrate describing the noise and leaves less for the picture. Scanned grain plates from real stock look better than the generated noise built into most editors.