A dark background is noisy, but the face still needs its fine texture. This is where noise reduction becomes a trade-off. A perfectly smooth still can look more artificial in motion than the original recording. Judge a short moving passage rather than a paused crop alone.
Establish a useful preview
Work with original media or a sufficiently detailed preview. Heavily compressed proxies can introduce artefacts that are absent from the source. Set playback quality so that the changes made by the filter are genuinely visible. A face, hair, fabric and an even dark area within the same shot make useful comparison subjects.
Distinguish changing brightness speckles from coloured blotches. Some tools offer separate controls for luminance and colour noise. If coloured dots are the main distraction, targeted processing is more sensible than heavily softening the entire image. Control names and processing order depend on the application and filter.
Begin with a small change
Start with low strength and increase it only until distracting noise becomes less noticeable during normal playback. Compare using the same display conditions and with additional sharpening disabled. An image can appear sharper because its edges are stronger even after its fine texture has disappeared.
Spatial methods examine neighbouring pixels. Temporal methods also use information from other frames. The latter can produce trails or smeared detail during movement. Pay particular attention to a hand crossing the body, a blinking eye or hair against a differently coloured background. These transitions often reveal problems more clearly than a static wall.
If the shadows alone are badly affected, investigate limiting the treatment where your tool allows it. Keep transitions soft and check that the selection remains convincing during movement. A complicated mask is not automatically an improvement over a weaker overall treatment that preserves the scene naturally.
Judge the finished format
Export a short section both with and without the filter. Watch it at the intended output size and an ordinary viewing distance. A difference visible only at extreme magnification may not justify much longer rendering or a noticeable loss of detail elsewhere.
A noisy but readable facial expression is often more useful than waxy, over-smoothed skin. If the source is also out of focus, underexposed or heavily compressed, noise reduction does not repair those separate problems. Consider another take or a shorter use of the shot rather than attempting to conceal every weakness with increasingly aggressive processing.