Digital grain added in post looks different from film grain, and most photographers can feel the difference even when they cannot articulate exactly why. The reason is structural, and fixing it requires understanding what film grain actually is.
Film grain is silver halide crystals that vary in size, distribution, and clumping behavior depending on film stock, ISO, and development process. It is not uniform. It is not perfectly random either — it has a texture and a clustering tendency that varies across the tonal range. Shadow areas in particular tend to show coarser, clumpier grain than midtones, which is the opposite of digital noise, where shadows are noisier than highlights.
The failure mode of most digital grain additions: uniform grain applied globally across all tonal values, generating a fine, evenly-distributed texture that reads as digital noise rather than silver chemistry. It sits on top of the image rather than feeling embedded in it.
Approaches that produce better results: apply grain in a way that varies by luminosity zone, with coarser grain in shadows and finer grain in highlights. Lightroom’s grain tool gives you size, roughness, and amount — roughness is often underused and is the control that most affects how organic the grain reads. For more precise control, Photoshop allows you to add noise on separate layers per tonal zone, blend in luminosity mode, and apply Gaussian blur at fractional radii to shift the grain character from sand to clump.
Overlay versus luminosity blend mode matters. Grain in luminosity mode affects tonal relationships without altering color, which is closer to how silver grain behaves. Overlay creates contrast-boosting interactions with the underlying color that can look appealing but are not optically accurate to film.
The grain should disappear at normal viewing distance. If you can see it, it is doing too much.