Few years ago I developed some code to simulate film grain, using an approach that is probably similar (but independent) to the one pointed out by Francisco @cribari: given a source image, the code generates a “grainy” version by literally adding one grain at a time, such that the average grayscale value is preserved locally.
I have not worked on this project since quite a while, and unfortunately the code is not yet in a shape that allows to make it public, but I might revise it is there is some interest (although it is REALLY slow on large images).
Nevertheless, I would really be curious to see how it compares with the other samples and methods that have been discussed above. So I put here some samples cropped from initial 6000x4000px images uniformly filles with solid gray:
50% gray, large grains:
50% gray, medium grains:
50% gray, small grains:
10% gray, large grains:
90% gray, large grains:
I can provide the hi-res un-compresses TIFF files if needed…