A fine, stable crack in a wall finish can often be repaired, but first make sure it is not growing or accompanied by distortion. Filling an active crack without understanding the movement usually means it comes back.
Mark and observe before filling
Take a dated photo and note approximate width and length. If the crack is new, diagonal, widening quickly, or appears with doors that no longer close, assess seriousness before cosmetic repair.
Remove loose material without over-cutting
Scrape away loose paint or plaster until the edges are sound. A very fine crack may only need a flexible repair compound; aggressively widening it removes sound finish without adding benefit.
Use tape or mesh when the repair system calls for it
On drywall joints or somewhat wider cracks, compatible tape can help bridge movement. Cover it with thin coats of compound, allow each to dry, and feather successive coats farther beyond the repair.
If it returns, stop treating it as purely cosmetic
A crack that repeatedly returns in the same place can indicate movement, moisture, or a failed joint. Before filling again, identify the cause and seek assessment if structural warning signs are present.