Creatures acting as cover

Half-cover is defined as a type of cover which obscures at least half of your body, and in the list of objects which acts as cover, it states that creatures can act as half-cover. 

My question is, does a creature act as half-cover passively? That is to say, if there is a creature between me and an enemy making a ranged attack at me, do I get the +2 bonus to my AC? Or, does using a creature as half-cover require you to declare that you are going to use your action to hide behind the creature? 

It’s passive, no need to declare that anyone is acting as half-cover. This also means enemies provide cover to you and vice versa.

Quote from Kaboom979 >>

Half-cover is defined as a type of cover which obscures at least half of your body, and in the list of objects which acts as cover, it states that creatures can act as half-cover. 

My question is, does a creature act as half-cover passively? That is to say, if there is a creature between me and an enemy making a ranged attack at me, do I get the +2 bonus to my AC? Or, does using a creature as half-cover require you to declare that you are going to use your action to hide behind the creature? 

As Saga said, it's entirely passive, and cover cuts both ways.

A little more specifically, cover doesn't just apply to ranged attacks either. If you're trying to attack a creature with a Reach melee weapon, while another creature is interposed between you and your target, the target still gets a cover bonus.

I allow my players to fire through friendlies without penalty as long as they give a heads up first.  Hostiles the half cover applies though.