I'm going to start out by talking about airplanes.
Pilots rely on checklists. There are checklists for taking off. There are checklists for landing. The checklists especially come in handy for bizarre flight scenarios that the pilot is unlikely to ever encounter -- specialists at aeronautical companies study those scenarios and determine the optimal set of steps for the pilot to perform.
For instance, there is a checklist for a pilot flying a Cessna when one of the engines blows out. And the specialists at the corporation studied the scenario extensively, and determined that item number one on that checklist is simply this: "Fly the plane."
That always struck me as beautiful: yes, something bizarre and confusing is happening, yes you're scared, yes the stakes are high, but your job is still the same, and it's still something you know how to do. Don't panic. Don't run screaming from the cockpit. Stay there. Yes, follow the rest of the steps, but first and foremost: fly the plane.
I think the analogous statement for an improv narrator is "Watch the show."
It's often said of improv that your understanding of the show increases as your distance from the stage increases. If you're the hero of a story, you have no clue what's happening -- you just hold tightly to your character, your point of view, and your motivation, and react to the constant barrage of things that happen. Other players in the story have a little more perspective -- it's not quite a confused mishmash of events, and they can make out the outlines of the plot.
Meanwhile, the audience always knows *exactly* what's happening.
Being a narrator puts you in that privileged audience position. You have a seat that gives you a nice view of the stage. You don't really have to get up, or move around, or play characters. You're less like the hero, twitchily grabbing onto every offer because it might be important, and more like the audience, sitting back and seeing the overall shape of things.
As a narrator, the best thing you can offer the show is this perspective. So, no matter what else you do, no matter what else happens, no matter how tempted you are to get lost in your own head: watch the show.
Armed with an audience perspective on the story, you have a great opportunity to feed that perspective back into the show. You can let the stage players in on things that are glaringly obvious to the audience.
So my next piece of advice, after "Watch the show," is this: "Say what's going on."
Obviously, don't be blandly literal about this. Don't say: "He reached his left arm forward, and then stopped, and then stepped stage right, and then scratched his head." When you watch improv, you're not a motion-sensing robot -- in your mind's eye, you're watching a story play out. If you already know that this performer is playing a detective, and he's in his office at midnight, and he needs to get his gun, then you're already *seeing* something more meaningful than a Logo program.
So instead, you say, "Sam looked for his gun. It was gone. That meant trouble."
This leads to something really fascinating with narration: if you watch the show, and if you say what you see, and you're not acting like a motion-sensing robot ("IMPROVISOR 3-B HAS TOUCHED HER FOOT BLEEP BLORP BLOOP"), you're invariably going to say "obvious" things that haven't occurred to anyone on stage. Those are the greatest gifts you can give the stage performers: when you're letting them home in on the story that the audience is watching.
In the example above, maybe the onstage 'detective' had no idea *what* he was doing. Maybe he'd forgotten that whole conversation in the previous scene about the upcoming shootout. Maybe he was just listlessly making random motions because he was stuck in his head trying to figure out what to do. He had no idea what you were seeing -- and, by extension, he had no idea what the *audience* was seeing -- until you told him.
I think of a narrator as just another performer onstage, but one with special abilities that the other players don't have. And so the question of "What should I do as a narrator?" becomes "What can I do for this show that the other players can't?" The first and most obvious one is, as above, "I can watch the show and report back on it."
But there are other useful things a narrator can do.
Most notably, a narrator can describe things.
Say that detective's office we were talking about has a comfy rolling desk chair, dingy hardwood floors, the sharp scent of cigar smoke, and a ceiling fan that buzzes and wobbles when it's on. In a movie, you'd get most of that (everything but the smell) from the mise-en-scène: you see the first master shot of the office, and you get all the visual information instantly.
In an improv show, you get none of that. In an improv show, all we have are the performers' actions, the things they say, and (ideally) sound and light offers from the technical improvisors.
Now, I'm sure some improvisors will read that and say, no, no, I can convey all of that information with movement and dialog. And that's true: it is *possible*. But doing that requires one of two things: skill or awkwardness. Yes, if you're Stephen Kearin, you can relax into a cheap folding chair with a precise physicality that implies "comfy rolling desk chair". Most people on earth are not that skilled, and the audience is stuck imagining the plain folding chair that they see before them.
Alternately, if you're willing to be awkward and stilted, you can convey those notions through dialog: "Hey, look! A comfy desk chair! Dingy hardwood floors! Do I smell cigarette smoke? Wow, that ceiling fan is buzzing and also wobbling when it is on!"
(For added fun, imagine that dialog if the detective is the only person onstage.)
I suspect many improvisors think this sort of explain-alog works a lot better than it actually does. True, audiences will cut an improvisor a lot of slack, since they're happy to have *any* kind of sensory details about the invisible setting, but it still takes the audience right out of the story. Sometimes it gets a laugh, just from being so glaringly fake.
And what's worse is that, if we're onstage talking about the setting, then we're not talking about anything important. We're not talking about relationships. We're not talking about each other. We're not talking about how we feel or what we want. Instead, we're talking about furniture.
Now, I'm sure that you're now imagining ways to talk about furniture that are meaningful and engaging -- "Yeah, I'm talking about the setting, but it's *really* about how I feel about my scene partner! I'm subtle like that!" This is certainly a noble thing to aim for. But it takes a lot of skill to describe the setting in such a way that you don't stall out the scene.
Again: awkwardness or skill. Choose one.
But this is where the narrator comes in. As a narrator, you can describe things without being awkward, and it doesn't require any skill whatsoever! Descriptions that make for awkward, painfully-forced dialog make for perfectly natural narration. And what's better, you don't have to be at all skilled at 'making things up'. All you're doing is following the same rules as before: watch the show; describe what you see.
If the show's going at all well, you will have some sense of what the setting is like. And if you describe what you're convinced is already there, you'll invariably throw in details that make perfect sense to the audience, while being a helpful surprise for the players onstage. They just walked into the basement of an abandoned house? Of *course* it's dank and unlit. Of *course* the floorboards are uneven. Of *course* the air is musty and damp. You don't have to invent anything new.
You can also describe the past.
Again, many improvisors may claim, "Hell, I can do that with dialog." And again, you *can* do that, but you're choosing between awkwardness and skill. Unless the past has absolute, obvious, direct relevance to what's happening onstage, talking about it is a surefire way to stall out the scene. As you talk about something besides the here and now, you start to lose track of the story, and soon you're lost in reminiscing -- which might be creative enough, but it is rarely, if ever, dramatic.
A narrator can describe the past very easily without any risk of throwing the scene off. This leads to a really convenient trick: when something interesting happens onstage, claim that it is something that *regularly* happened. This is especially handy for establishing characters -- I can't count the number of times this paid dividends in Dickens. In one show, Curtis played a father who, early in the show, gave his son a piece of lukewarm praise, I established that it was always his manner to tepidly appreciate things, and he was off to the races with a sharp character and a delightful game to play.
Lastly, there's a special case for improv formats inspired by novels and short stories: a narrator can embody the prose style of the source material.
Face it, a lot of what makes a Lovecraft story *feel* like Lovecraft is the way that writer uses language -- the same goes for Dickens, or Nancy Drew stories, or the Arabian Nights. If you imagine a Lovecraft novel written in the style a Nancy Drew book, or Dickens written in the style of the Arabian Nights, you wind up with something that's radically transformed from the original. A narrator can be a way to retain how the books feel.
Sadly, I don't have good advice for that. The only thing that works for me is to just read tons of the material until I get the voice of the prose. You want to get to the point where you don't have to *focus* on the prose style. Instead, you put your focus where it belongs, on the show.
Of course, there are exceptions to everything. There will be shows where the onstage performer points downstage and says in terror, "Look! Look at that thing!"
And the other performer responds, "What *is* it?!"
And then they both steal a furtive glance at you and quietly wait.
Okay, sure, at *that* point it's your job to invent something from scratch. But your default mode -- where you'll spend most of your time as a narrator -- is that you're just a talking audience member. You're reporting back on what you see, in your mind's eye, as you look out on the stage. Above all, you're just watching the show.
 ... and if it's a confused, meaningless mess, they know *exactly* what kind of confused, meaningless mess it is.
 By this logic, the baristas in the downstairs café understand the story on a level so profound that it's like they aren't even aware the story exists.
 That said, I would certainly prefer stilted dialog like that to no sensory details whatsoever. Without a narrator, clumsy descriptions are often the best you can do.