Caravan Guard says (+1 Action has no effect if it's not your turn.)
It has been previously discussed whether that phrase means "you can't do anything with the extra action if it isn't your turn" or "you ignore the instruction to get +1 Action". I don't think the question was answered, as it never mattered.
But now, it can matter. With Black Cat, Way of the Mouse, Diadem, and Storyteller or Black Market, you can do all sorts of crazy things when it isn't your turn, including playing a Diadem and buying a card while it isn't your turn.
So do you automatically always have 0 actions while it isn't your turn, making Diadem always worth ? Or, do cards you play that give +actions work normally? Caravan Guard is the only +action card that has that reminder text; one could even argue that Caravan Guard doesn't give an action, but other +action cards do.
And if it is possible to get +actions when it isn't your turn, I assume that you start every turn except your own with 0 actions?
This is a great question.
I hope it's ok, I'm going to map out to a specific example:
Player A buys a province.
Player B reacts with Black Cat, plays as Way of Mouse - Vassal. Discards a Village, playing it. (+2 Actions)
Player B then reacts with a 2nd Black Cat, plays as Way of Mouse - Vassal. Discards a Black Market, playing it.
Player B plays a Diadem.
Is Diadem worth $2 or $4?
And if earlier in other turn, Player B had reacted to an attack with a Caravan Guard, is Diadem worth $2, $4, or $5? (since Caravan Guars is the only one with the parenthetical)
(and this is all assuming you start with 0 actions on other players' turns, which I think is a fairly safe assumption)