eminem definitely wasn't in on it.
Oh come on, in this sue happy nation, you don't do anything more than a roast without least getting all the legal bullshit out of the way, even if that means convincing the 'proposed victim' to go along with it. MTV isn't that stupid to do something unplanned as that. Besides you can clearly see in the video that he seen it coming for a long enough duration to get the fuck out of the way.