Scenes are "smaller" than events. They always have to happen as if they are literally a scene in a film, with characters and action described like you're watching a scene in a film or reading it in a book.
(Notice here that I'm not talking about dictated scenes specifically. The difference between a dictated scene and a normal scene is only how its question gets answered. The size and scope of a scene, and when you should make a bit of history a scene instead of an event, is the same regardless of whether it's dictated or not. If what you're picturing couldn't be a played scene, stop: you should either be making it an event, or looking elsewhere to make history this turn.)
Events are larger in scope, and don't reveal the amount of detail that a scene does. Although an event should be described clearly enough that the other players can picture what has happened from a birds-eye historical-book view, describing moment-to-moment action and motion in an event is inappropriate, and an event is not limited to being one moment in time.
When you already know what you want to add
There are pros and cons for choosing each in terms of playing strategy, but if you already have an idea of what you want to happen, there will always be only one right choice. Ask yourself: could multiple events happen within the timeframe I'm thinking of? If yes, it's an event, if no, it's a scene.
Consider a few examples:
A piece of history where there's only one right answer to "scene" versus "event": The starship Endymion's first voyage.
Many scenes could happen within this (the christening ceremony, the swearing-in of the captain on-deck for the first time, an emergency occurring during this first ceremonial cruise, a romantic encounter between shipmates, etc.) so it must be an event.
A piece of history that could be either: The starship Endymion's christening ceremony.
This could be a scene or an event, based on what you want. Do you want room for multiple happenings during the christening ceremony? Then make it an event. There could be the christening itself, the after-party with political maneuvering and schmoozing, and so on, multiple scenes within this one event.
But it could also be a scene, if you want it to be. If you make it a scene, then maybe it's only the christening itself (already mentioned above as a possible scene), or only the after-party, etc. Of course this needs an event to happen within — maybe a war or the beginning of an exploration project.
A piece of history that can only be a scene: Why did the Consul accept a bribe during the christening ceremony of the SS Endymion despite his widely-known reputation for personal integrity?
A scene is for when you have a very small detail that you want to add to the history. If you don't want to dictate what the detail is, then you make it a normal played-out scene. If you do want to dictate what the detail is, then you make it a dictated scene. The point of a scene is to focus on that one detail and the circumstances that surround it. If you don't have a detail in mind, then it shouldn't be a scene. (And if it's too small to fit multiple scenes into, then maybe it shouldn't be an event either and you should look elsewhere in the history to add a piece of history.)
When you don't already know what you want to add
Here, you're not starting from an idea of exactly what you want to add, so the pros and cons of dictated scenes versus events, in strategic play terms, become relevant.
Which you pick depends on your motives.
Do you want to add a bit of colour to the world/universe, or do you want to establish that an event occurred? Naturally for the latter you should make an event; for the former, a dictated scene.
Do you want to draw the attention of other players to this bit of history with the hope that they'll invest in the idea you're adding? Then it should either be an event (so that they can engage by added scenes to it or events before/after that relate to it), or a played scene (so that they engage directly by participating). Dictated scenes are poor tools for this motive, because though they might pique the other players' curiosity, it doesn't have a high chance of engaging their play with what you just made, unless you picked an happening and people that everyone feels very strongly about. (That latter tends not to happen by design though, more often by accident, so its rarer than you might think.)
Do you want to have a large influence on the period and history overall, or do you want to establish ground-level details about just this period's people and places? For the latter you want a scene, because only scenes show us what life and places are like, while events leave the details unspoken and for later to establish. If you want larger influence, an event makes a large mark on a period, and can potentially echo through the history, either by what it implies about other times in history, or by capturing the imaginations of the other players so they build on it there and elsewhere.
There is good advice (and it's similar, now that I review it) on page 66, "How Do I Make a Good Question?":
There are generally two reasons you'll make a Scene:
- There's something specific you want to know about the history, so you have a particular Question in mind.
- You want to get the action rolling, do some role-playing, and immerse yourself in the setting.
If you don't want to do some role-playing and setting-immersion solo, even briefly, you shouldn't be making a dictated scene, but rather an event (or, again, looking elsewhere to make history).
Dictated Scene versus Event
There is a theme throughout: events are large, scenes are small. There's a lot of scale gap between them, and you should always take the opportunity to emphasise that scale gap to best take advantage of the different ways they can influence the game and other players. When you make an event, go big — as big as you can while still zoomed in correction on the idea, and small enough that it isn't infringing on the scale that belongs to periods. An event can have an implied scene (p. 68), but should always have room for many scenes related to the event or happening within its span of time.
When you make a scene, zoom right down into a single moment of time, a specific cast, and one singular question that the scene turns on. Get as tightly-focused as you can when you zoom into the scene level.
Recall or re-read the advice in Microscope about history-piece size
Properly scaling your periods can go a long way to making the distinction between events and scenes clearer, too, because a right-scaled period gives room for events to breathe. Review pages 20 and 21 for general advice on scaling your periods, events, and scenes, and the specific advice for each one's scale on pages 22, 24, and 29.
If you're having trouble with scenes versus events, your periods (and maybe your history) are probably too small. You can make a tightly-focused history, but that makes it much harder to play well when you're not fully confident with the game yet. Make sure your periods are large. Very large. They should always have enough room inside them that you can't imagine a limit to how many events could be placed inside it.
If your history is the scale of a galactic empire, your periods could be eons of time, centuries long. This is the kind of scale you want when you're just getting started learning to play Microscope. It's a learn to walk before learning to run, sort of deal.
Once you feel confident playing the game and using its rules to make awesome evenings of entertainment, then you can consider the uncharted challenges of making smaller history timelines work nicely, where events and scenes and periods all rub shoulders and it takes skill to keep them properly separate. Then, if your overall history is the scale of (say) a world war that could go on for years or decades, then your periods should be months or years, with time for many victories, losses, developments, and general turns of fate, but your events could also be the scale of weeks or months, and how to keep them separate is going to require the "feel" for what's right that you got from previous play experience with easier-to-manage timelines.
Make sure your periods are long enough, and you'll have room for events that don't crowd your scenes, and it will be easier to tell the difference between them.