hairline cracks on both walls and ceiling?

Could be one of a few things if in straight lines on new plasterboard it's movement within the boards plasterers use scrim mesh on joints and internal angles and this isn't a cure for cracking it's only a preventous measure taken although all cracks should be scrimmed movement will crack any plaster , If plaster is cracking everywhere the likeness is that it's not adhered to the backing surface and will need taken off and prepared in the proper manner

Could be a case of not bonding to the previous substrate or incorrect prep ie no pva or grit used. Does the backing sound hollow when you knock on it? If so that would suggest that some parts of the plaster have blown. There could be a multitude of reasons for hairline cracks appearing after the plaster has dried out. You'll get all kind of answers on this one because we are all guessing as to what was and wasn't done. It may help to see photos to better identify the problem