There's an implied cost to some action cards. This cost does not actually prevent you from playing the card if it's not paid; it just does not get the full benefit.
For example, Tactician states that you discard your hand. That's a resolution of the card and not a cost at all. If you discard at least one card using this method, then you get +5 Cards, +1 Action, +1 Buy, +1 Partridge in a Pear Tree. Technically speaking, discarding at least one card is not part of the cost of playing Tactician. You can always play it even if it gives you an empty hand afterwards.
Same thing for cards like Trading Post. You can play it without two cards in your hand, but if you do, then you don't get the Silver.
So, Ikari is right that action cards don't have additional costs other than -1 Action just to play them. Some cards do have other costs to get more out of them, though.