It’s very difficult to prove a negative, but I am reasonably confident that no rule in 3.5 or Pathfinder explicitly states that all armors of a given weight class slow you down by the same amount. In 3.5, at least, there were certainly a few super-heavy armors that reduced the speed by more than your typical heavy armor (Races of Stone had a couple of these).
So yes, every Medium or Heavy armor must somehow explicitly state its own speed reduction, even if the overwhelming majority of them have the same speed reduction. On the other hand, if a given case failed to do so, I’d just chalk that up to the designers not realizing there was no such general rule, since it usually is so consistent and armors which slow people more or less than others in their category are quite rare. It’s also worth keeping in mind that it is consistently weight category that we should consider, not sheer weight.
Let the players make up all that stuff!
How about letting the players fill the rest of the exposition? So following from your quote:
Wilma: (shouts) "I'll cover you. GO!" I turn dive behind the garbage cans in the alley, drawing my gun. That first goon is going to have a nasty shock. The rest will get suppression fire.
Craig: (over comms) "Debbie, turn left on the freeway! We'll hijack a car."
Debbie: (over comms) "No, we're close to Clown territory, if we get there we can get backup and come back for Wilma. Follow me!" I jump and head straight for the backstreet with the Clown markings.
Here you are allowing the players to come up with interesting ideas about what and how things unfold. You can then incorporate whatever was made up into your game.
I started several games just like that: No one had characters, states, or background: just names and a situation. Things moved on from there as they described themselves, what they were doing, and what skills they had.
The main rules we had were don't be a dick and never knowingly contradict someone else's story. These are fairly simple really. Also, because you are creating the world as you go, whatever you say is what happens. So, it's best to leave things open ended and not closed. So, in your example, what would be the point of Wilma saying "I take out my nuke grenade and blow the alley so the buildings collapse(1)." In that respect, an idea of what style and themes you wanted to play helps: is it a gritty noire cyberpunk setting or a super hero show a la Arrow/Flash/Daredevil?
One of the games we ran several times started with
You are at a wake.
That was it! No setting, no 'nuffing. Everything was build up from the ground up. Try it next time you have a couple hours to kill with your group.
As mxyzplk stated in a comment:
This gets to the heart of it. A real narrative game is one where the
exposition isn't required because it's collaboratively generated.
Tacking a single narrative technique on an otherwise simulationist
game will be more problematic.
The first point is indeed spot on.
As to the second point, it still work even in a simulationist game if your idea of role playing is building a collaborative story where both GM and Players have input as to where the story goes and generate content on the fly. On the other hand, if the GM does not allow this and/or treat the game as a "GM vs Players", then in medias res cannot work as the players are denied full knowledge of the situation. Clearly, if the characters lack said knowledge, that would work as Nvoigt's fine answers suggest.
(1) Although, I can think of a fun game based off that too!
Best Answer
I've used this technique sparingly in the past. Sparingly, because
When I do use this cut-away technique, I typically stick to verbally painting a single visual “shot”, or describing one brief exchange between NPCs (like, a sentence/action or two each). I will sometimes give away new information like this, but only if I want the players to metagame with it. I will more often use this to emphasise something the PCs or players already know or will know shortly, or to skip time while also giving them info that they would have gotten if we hadn't skipped ahead.
For example, I used this once to describe an aerial view of a city during a riot, at the very end of a session. The PCs and players already knew of the chaos, but this let me show them fires breaking out and confirming their suspicions about how widespread the activity was. I could have done this by devoting a lot of PC eye-level time to moving around the city and showing them, or by having NPCs come to tell them about the extent of the riot, but with two sentences I could take a short-cut and we moved the action forward more quickly, handwaved how they actually learned this information, skipped a bit of time, and set up a clear understanding of the situation to begin the next session with.
Like most ways of breaking the “rules” of storytelling, it's far more powerful and effective when done rarely and judiciously. I can probably count on one hand the number of times I've used this technique in games that aren't frequently narrated in this way already.
So yes, you're certainly allowed, but my advice would be to either 1) do it very rarely, saving it for when it will have the most impact and feel the most “right” to your audience, the player group; or 2) do it often, using it to change the “style” of the storytelling and metagaming standards of the campaign.