For example, say a card reads: "Take a horror, then do X"
Does the Investigator take a horror, then stop to wait to give the keeper the option of playing an trauma; after which he may continue with "X" after the trauma card (if played) is resolved? Or does the keeper wait until the entire action is resolved before he has the option to play trauma?
This seems relevant in the case that the Trauma could impact what the investigator can do with "X"... namely if "X" is dependent on location and the Trauma moves you.
(The particular card I have in mind is Carolyn Fern's OPG ability "Hypnosis" from Forbidden Alchemy.. but I'm sure it's applicable to other cards in the base game as well.)
Edit: An example from the base game could be Jenny Barnes OPG which more or less reads: "Action: Take two movement then an attack action...". It's possible for her to take a horror in transit to the monster and it would feel as though the keeper would then have the opportunity to respond with a trauma
As stated above, the trauma card is handed immediately after receiving horror/damage. So it will interrupt the action (which may and may be not continued afterwards). To be specific, if Jenny uses her OPG ability to move twice and attack a monster, when she fails horror check and receives "Panic", she will be moved three steps and her ability will be (presumably) mitigated
In the particular case of Carolyn Fern, we've always played that the horror is given and healed simultaneously (so it can't be broken by Panic), but that Dr. Fern can't heal her own horror. That's not what the card says, but I think it makes for the best gameplay.