Ideally, a moisture barrier/breathable membrane should sit on top if the rafters and held in place by vertical battening, tile battens then fixed etc. Only worth doing if the roof is to be replaced. It’s important to retain an air gap so insulating between rafters and then plasterboarding direct isn’t ideal either (and the spacing is unlikely to be right for the plasterboard joints.
It would be better to use a high tech multilayer insulation stapled to the rafters and further fixed with vertical tile battens. Then build a frame (demi-chevron) between the purlins with 60 cm centres to fix plasterboard to.