What was odd is how entirely my GMing style changes when I switch from a long-term campaign to a one-shot or focused mini-campaign. When I'm running a campaign, almost all my interactions with the party will be either as omniscient arbitrator or else first-person representation of a non-player character. I role-play the NPCs. Keeping the interpersonal interactions detailed, plausible and entertaining is my main strength as a GM.
What suffers is my plotting. My games wander and meander, depending on which NPC the PCs go talk to and what they say. There can be no railroading (a general plus) but there is also usually darn little structured tension and resolution.
So, I tried some structured games. One-shots have to be structured. Four hours is not enough time to take a Method approach to gaming. The objective has to be stated clearly, the obstacles delineated, and the PCs set on the path to getting past the obstacles to the objective.
When I do this, all of my NPCs suddenly become third-person. "He says..." "She tells you that..." I'm so busy running through the plot that I lose all sense of connection to the NPCs. I feel like I'm rushing them, railroad-style, through a series of plot points. I feel off-balance and self-conscious, like I'm not doing it right or well. But some of my players have told me that they enjoy this style as much as (or in some cases more than) my usual style.
The strange exception was the entire pulp campaign. It was structured as a series of loosely-connected adventures, each one more or less plotted out. But there was still enough slack for the PCs to bounce around inside each adventure a bit, and I didn't have a hard time dropping into character for the NPCs. I'll have to think about that.