So, according to the rules of reference book, there is an action window after the staging step. The rules also say that if a "when revealed" card comes out of the encounter deck you have to resolve it before drawing another one. In that scenario, my question is the following: How can I play an action that for example cancels the "when revealed" effect of the first card without drawing a second one (in a two-player game) in order to complete the staging step (since the action window is after that step)? Can you in those cases prematurely play actions after the first card although the staging has not yet been completed (namely the second card has not been drawn and resolved)?

The thing with cards that allow you to cancel when revealed effects is that they are usually responses, not actions. So it would look like this:

Reveal a card

Play Test of Will (I assume this is the card you are referencing, but Eleanor or The Door is Closed also apply) as a response. Imagine this as interrupting the normal game flow, as opposed to actions, which have designated framework times when you can activate them.

Continue revealing cards for staging until resolved.

True. Didn't think about that. So there is no case where you would need to play an action between encounter cards during stage, right?

It's unlikely that you will be able to play an action between encounter cards, you will have to play actions before or after staging. However, there is one relatively common case where you can play an action during staging -- if the encounter card causes an immediate attack to be made, action windows will be open between the steps of the attack.

One things that I'm not sure you understand is that action window is... for action. You may remember it quite easily ^^. Response don't need any action window, they only need the thing that make them trigger, in that case a "when revealed".

Thanks everyone! This helped!