In T13 each Facet has a Tone, the Tone can affect the way the game is played. A Miasmic Horror Tone favours creepy story-telling and creepy, terrifying actions, it is not the time for a couple of characters to try and joke their way out of trouble.
You can find a list of all the Tones here.
It is normal for the Ref to assign Scenes and Stories certain Tones, but some Character actions may also alter the Tone (especially powerful Annexes and certain Nimbeds).
Generally speaking if you are acting in accord with the Tone you receive a +1 Success Level to every action, if you are acting completely against the Tone (using the opposed Facet for example) then you will receive a -1 Success Level to any action. So if the Tone is Horror then attempts to Wound, kill and scare are increased a Success Level, but an action to heal a wound, or cure a disease (Phoenix actions) would have -1 Success levels.
As always, the Referee is the final arbiter of all the action in their Story, so if they say your action is against Tone, you will have to take the negative success level. It should be noted though that the Ref might just have given you a clue as to what is really going on.