If an event says to discard a card type, like Watcher in the Water says: discard a shield or roll the dice; can you discard a wild instead (or any brown card for Frodo) or does it have to be the type mentioned?

Watcher is discard Hiding or roll a die, you can use a Wild instead of Hiding here, as in pretty much all the cases. Frodo can only use any brown Quest card as a wild, can't use brown Legendary cards as anything other than what they are printed with.

Ah, thanks.