From [Dune][1] by Frank Herbert:

>"I must not fear. Fear is the mind-killer. Fear is the little-death that 
brings total obliteration. I will face my fear. I will permit it to pass over me 
and through me. And when it has gone past I will turn the inner eye to see its 
path. Where the fear has gone there will be nothing. Only I will remain."

>He felt calmness return, said: "Get on with it, old woman."
 
>"Old woman!" she snapped. "You've courage, and that can't be denied. Well, 
we shall see, sirra." She bent close, lowered her voice almost to a whisper. 
"You will feel pain in this hand within the box. Pain. **But!** Withdraw the hand 
and I'll touch your neck with my gom jabbar --the death so swift it's like the 
fall of the headsman's axe. Withdraw your hand and the gom jabbar takes you. 
Understand?" 

>"What's in the box?" 

>"Pain."

Is the sentence "But!" considered grammatically correct? How is this meant to be interpreted? How is it different from using a comma? It should be noted this is part of a dialog. 


  [1]: http://johnnyturbo.org/dosh/02%20-%20Herbert,%20Frank%20-%20Dune.pdf