Fine hairline cracks in plaster are common and usually cosmetic. Cracks worth investigating are those that are wide, diagonal, visible on both faces of a wall, tapering, stepped through masonry joints, or clearly widening over time — especially when accompanied by sticking doors or sloping floors.
Most houses develop some cracking as materials dry out, shrink and respond to temperature change. These cracks are typically hairline, straight, confined to plaster finishes and stable over time. They can be filled and redecorated without further action.
Cracking that reflects structural movement tends to follow the lines of stress in the building: diagonally from the corners of doors and windows, in a stepped pattern through mortar joints, and often through the full thickness of the wall so the same crack appears inside and out. Width, direction and progression matter more than length.