Lets say:
It's Andy's turn and he pays three coins to assasinate Bob.
Charlie has a Contessa and wants to disrupt Andy.
Is Charlie allowed to use his Contessa here?
Lets say:
It's Andy's turn and he pays three coins to assasinate Bob.
Charlie has a Contessa and wants to disrupt Andy.
Is Charlie allowed to use his Contessa here?
When Andy has his turn, the following happens:
A similar effect happens with someone claiming Captain. On the other hand, anybody may claim to have the Duke in order to block someone's Foreign Aid.
Source: The game's designer, answering pretty much this exact same question on BGG: https://boardgamegeek.com/article/10440845#10440845
No, only the person who has the action taken against them can block it. It doesn't Matter if it is assassination or the stealing of coins, the target will need the card (or claim to have it) to block it. The only thing another player can do is challenge the acting player if they don't think they have the card they claim they do.