This question of GM narration seems to crop up a number of times. Check out:
http://apocalypse-world.com/forums/index.php?topic=2796.0He seemed to encounter a similar problem in his AP.
One trick for offscreen cameras is to describe a narrative focus shift. Let me provide an example:
P1: I slice the wolf open!
GM: That sounds like a hack and slash - roll + STR!
P1: 9!
GM: The wolf latches on your arm, and while you strike it with your sword (whisper: roll damage) it clenches it's teeth, and paws at you (you take d6 damage).
Marlow (P2)! You hear the chewing of jaws on arm-bones while sidling into the shadows, and the sharp yelp of your friend as wolf-teeth hit bone. What do you do?
Also keep in mind that the concept of a skulking thief isn't necessarily inherent (stealth is a great way to surprise someone, but it's tough to stealth in a 10/10 room). Avoiding combat extends your life expectancy (significantly), but you can fight alongside someone who opens up an enemy to attack (such as with a defend shield slam) which is how you can slip a backstab in.
Hope that helps!