These are are intimately related:
1. Pacing. The GM is responsible for setting the pace by introducing new obstacles or by saying yes. The GM also gets to decide which scenes get resolved in which order if there is a question.
2. Scope. The GM is responsible for determining the scope of a proposed task. The GM can decide that multiple tasks are actually necessary to fulfill an intent (i.e., Stealth alone isn't enough to sneak into the tower; you need Stealthy to go unseen and Climbing to scale it), or that one test is sufficient to cover a broad swathe of story. The GM can also determine that a proposed task is not appropriate to a given intent.
I corner him and stab him in the face!