This question came up in my mind concerning my work while responding to a thread about writing humor. I have a scene in my book where an old man who is supposed to be helping my MC turns around in the form of a pig faced demon. It is not the climax, just a scene that confirms the MC's thoughts that he is dealing with something bigger and darker than he. The quick revelation is enough to send the MC falling backwards and I want some of that feeling conveyed to the reader. "Boo!" doesn't really translate well into reading/writing. I have read my passage several times and think that I am completely missing my objective. I attempt to describe what the creature looks like, but I think in the description, the shock value is weakened. Is it generally better to just show action and hint at physical attributes to ignite the reader's imagination?