One of the most important steps in compositing that is sometimes overlooked by new artists is matching the film grain of your integrated element to the plate. Actual film grain is created because of the random layout of the silver halide crystals on the film stock, digital “film grain” (sensor noise) is mostly a result of digital sensors being imperfect. Both of these create random noise patterns that change every frame. If our composited elements don’t have this form of noise that changes every frame they’ll stick out like a sore thumb! Moreover, if the noise pattern doesn’t match the same will be true.
Up to now I’ve been using Natron’s included SeGrain node but recently I ported SpinVFX’s Grain_Advanced which is my personal tool of choice for manually matching or adding film grain, it features individual blur controls for each channel and per-channel controls to control the grain in the dark and light parts of your images. It’s also got an aspect ratio control for matching anamorphic footage!