You need to be really honest in Narrative 40K missions. Because there are secret Victory Conditions and twists that only one player is aware of, when those twists or Victory Conditions are met - you need to speak up. Even if that means painfully telling your opponent they have just won!
Narrative 40K missions can only be played once each. Because of the way they are structured, once you know the twist or how the Victory Conditions play out for each player, playing the a mission again is hard to do with the same level of fulfilment. Narrative 40K will never give you the same mission twice.