You do not have to Set a Quickplay Spell Card to use it in your own turn - just the same as you do not have to Set your regular Normal Spell Cards before you activate them. You can chain a Quickplay spell from hand too, as they are spell speed 2 - that is, you don't have to set them if you want to chain them to something. So if it is your battle phase and you attack, and the opponent uses Mirror Force, you can chain My Body as a Shield from your hand.
However if you want to use them in the opponent's turn, you do have to set them on the field.
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You can only do so in your turn. You may do so during Main Phase like a normal Spell Card. You can also do so to start a Chain. For example, if your monster attacked and your opponent activated Mirror Force, you can Chain My Body as a Shield from your hand. If you plan to activate a Quick-play Spell Card during your opponent's turn, however, you must Set your Quick-play Spell Card during your Main Phase.
No, you can't. You can only activate a Quick-play Spell on your opponent's turn if you have Set it on a previous turn.
No, it doesn't end the battle phase.
I'm asuming you mean in the game "magic the gathering". If so, then yes. Quick play spells can be used during any stage of the game.
no, because for one there is no such thing, there are counter traps but they are not quick play