The Wall gives characters the nights watch trait and Baratheon affiliation.

Am i correct that this will only effect cards in play and not in discard, dead pile or in hand ?

Thanks

You are correct that unless a card specifically says otherwise, it only works on cards that are in play. Since neither version of The Wall (one gives Bara characters the Night's Watch trait, the other gives Night's Watch characters the Bara affiliation) says anything about characters in hand, discard pile, etc., they only work on cards that are in play.