I'm looking for a fic, but all I can remember is one scene.  I think Jensen has been hurt or sick for a while, and they have been filming Supernatural with Dean missing for a couple of episodes.  A new scene is being filmed where Bobby is supposed to suddenly show up and help Sam with something.  However,  Jensen is recovered and they replace Bobby with Dean without telling Jared to get a genuine reaction out of him. 

That's all I remember.  Hopefully this sounds familiar to someone.  Thanks in advance,




ETA: Found in comments.