i mean like they can play them when-ever they want to i think. out-side of maybe equipment-cards, e.g. de-spell which yugi used against pandora can only be used on ur own turn without being able to chain-effect or anything in-real-life but in the series yugi activated like a fast-play i think i think.
Yeh in the manga and the Battle City anime it seems all spell cards can be activated on the opponent's turn. IDK about the Dartz or Grand Championships filler arcs though since those were written by the animu staff whose job was solely to advertise the OCG.