If, for example, you have Peter and Mary run out of the rain into the 7/11, we can probably-- as long as you tell us it's a 7/11-- envision the store (though do make sure we have information about whether it's day or night). However, from what we've been told, the store has three people in it now: Peter, Mary, and the store clerk. (We expect the store clerk to be in there, so if he/she's not, let us know quickly so we can re-adjust.) If Mary goes to pay for her cigarettes, and Peter's daughter suddenly says, "I thought you quit smoking," we're going to be disoriented. Isn't Peter's daughter still out in the car? (Or "Peter has a daughter?")
Surprise can be fun in stories. But this sort of accidental surprise isn't. It breaks the "fictive dream" of a story and particularly ruins the experience of the scene. Think of it. As you read, you try to build the world, build the scene, right? You assemble the information that the writer presents and make a setting out of it, and people the setting. So there's a car, and there are Peter and Mary, and there's a parking lot, and it's raining, and they run in, and it's a 7/11.
Now sometimes there'll be new information that you didn't get earlier, like this is one of those super 7/11s with produce and a deli. And usually if the new info is presented right, even if it's a bit delayed (like you learn that not when they enter, but when she goes to get a cup of coffee and is presented with a super 7/11 array of six different coffees), you quickly adjust the experience to allow for that. But sometimes, the new information is jarring, and you think, "Huh? When'd that happen?" And the dream is broken, and this is just a book again, just ink on paper or pixels on the screen, not Peter and Mary and the rain and the 7/11 at all.
So how do you know what is new info that the reader is elastic about ("Okay, no problem, six kinds of coffee, super 7/11"), and what is new info that will jar the reader right out of the dream ("The daughter? But I thought she was asleep in the car!")? And how do you protect against that, or set up so there's no "jar" at all?
Well, you know, I think part of the issue is whether we're supposed to be in this with one character, in a character's point of view. And if so, what would surprise the character not at all or mildly (six different coffees when I thought this was just a regular 7/11) also won't have much effect on the reader. But what would presumably really surprise the character (if in fact she was in this store in this scene) will jar the reader. Would Mary be surprised to find the daughter at her elbow, dissing her about smoking? Yeah, if Mary left her asleep in the car! "Where did you come from? What are you doing here?" Well, the reader has that sense of shock too. And if you want the shock, I think Mary has to experience it.
... "I thought you quit smoking."
Mary turned suddenly and there was Emma at her elbow, barefoot and clutching her blanket. "I thought you were asleep in the car! Does your dad know you got out?"
If Mary reacts with surprise, we relax, because it was MEANT to be a surprise. We weren't meant to know that Emma was in the store too. We didn't miss anything.
But if Mary doesn't react with surprise:
"I thought you quit smoking."
"I did." Mary dropped her change into her purse, and her cigarette pack into her pocket. "This is just to prove to myself that I can resist temptation. Did you find the beef jerky you were looking for?"
Well, then we're going to stop and look back and try to find where in the scene Emma got out of the car and came into the store, because clearly Mary not only knows Emma was there, but that she was looking for beef jerky. That wasn't in the information base that lets us start building this scene, and so we go looking back for it, and if we don't find it, well, there goes our dream and your credibility.
This might seem like a minor issue, but it's not. It goes right to that "suspension of disbelief" required for the reader to get immersed in your story. If you want us to believe that these are real people in a real world, you have to be careful not to propel us back to the real real world by making this world seem like it's made of paper and ink.
This is, I think, even more paramount when the reader is (as most are) accustomed both to real real life and the "real life" presented in film and TV. In real life, when we stop at a convenience store and leave a child sleeping in the car, it's something of a conflict. Do you wake her/him up (and you'd know if it was her or him, of course) and carry him/her drowsing and protesting, through the rain to the store (and then how do you manage to carry both child and package of purchases?), or leave him/her sleeping peacefully in the car for the three minutes, during which time of course the car could be hijacked with child inside and even if it turned out okay (as it usually does on the news) you will regret it all your life? You think I'm joking? This was a constant conflict for me when my kids were small. It's not something any parent's likely to think negligible.
In real life also when we enter a store we quickly, automatically, size it up visually. Will it have what we need? Is there a long line at the cashier's? Does it smell like sour milk (in which case, can't buy the milk)? Does it feel safe? How many people are loitering in the background? Can we abide, even just for a minute, the music blasting out of the cashier's CD player?
A film or a TV show has no smell or feel, of course, but it has video and audio. So when Mary (Anne Hathaway) and Peter (Ewan MacGregor) emerge from their Prius and cross the wet parking lot, we hear the car doors slam and the raindrops splattering on the asphalt, and we see the neon reflections in the puddles and we see her putting her purse over her head to keep the rain off, and we see him sprinting to get the door first so she will think he's a gentleman, and when they enter the store, we see the young cashier's instinctive glance over (any customer could actually be an armed robber) and we see the first rack has the bright wrappers of candy and we see the flourescent light getting soaked up by the scuffed vinyl floor tiles, and we see the truck driver at the coffee stand filling his travel mug, and we hear the Muzak and the voice the teenaged boy is trying to deepen so he can get away with buying beer. Couple seconds, and our eyes and ears record all that.
Now let's say that a minute into the scene, Mary goes to the cashier and requests a pack of Marlboros, and Peter's daughter Emma pipes up in that insufferably wise little-girl-in-a-film way, "I thought you quit smoking."
Given that we SAW Mary and Peter dash into the store, and we looked around the store, and Emma was nowhere in sight... well, the "continuity editor" or whatever she's called is going to say (while they're still filming), "Hey, got to get Emma out of the car and into the frame." Too right. CUT!!!! TAKE TWO! So rewind. Mary gets out of the car and dashes into the store. Peter is slower and gets more wet, because he stops at the back of the car, opens the door, unhooks Emma's safety belt, and picks her up and carries her into the store. (Can you see it? Am I the only one who goes "awww" thinking of Ewan M cradling a sleeping child, bending his own head to protect her from getting rained on?) And then she wakes up and he puts her down, and we see all that, and so it's not a big jarring moment when she goes up to Mary and makes that carping comment about smoking.
So... so... we're fiction writers. We don't have a camera! And we also don't have someone else in charge of continuity. We have only words to express our consciousness about this scene, to give the reader the experience of the characters. So the question becomes: How closely do we narrate? How much blank can we assume the reader will "fill in"? How much is too much? When does too much detail become obnoxious?
Now I tend to over-narrate, I admit. Every step, every raindrop, every door slam. But you probably don't have to be that fastidious as long as you anticipate what the reader needs to build the scene and fully participate. What's important? Yes, it's important how little Emma got from the car to the store. Yes, it's important it's raining. Yes, it's important that it's a 7/11 and not a Safeway. Yes, it's important that it's night. No, the car model isn't all that important, and it's not all that important which of them first opened their car door. But it is important that it was dark outside and bright inside. And it is important that Peter carried his daughter while outside and only succumbed to her protests to be put down once they were inside.
There's no substitute for "sitting in the scene". Yep. Experience the scene from inside the POV character and narrate what's important, what the character notices. AND THEN GO BACK AND MAKE SURE YOU HAVE IT RIGHT! For example, maybe you want Mary actually to be surprised, thinking that Peter left Emma asleep in the car, and here she is, snarking about smoking. Okay. But if Peter had brought Emma in-- even if Mary didn't know it or see it-- is there any "rewind" necessary to set that up? Even the slightest fill-in might make this more experiential for the reader. Mary gets out of the car, slams the door, runs through the night rain and flings open the store door... and as she enters the moist, bright store, she hears but doesn't really register Peter's door slamming, and then the squeak of another door. Or maybe just a door slamming. Something to allow a subconscious hearkening back to the car, see. Mary might be inside now, but outside still exists, right? And the car still exists, and Peter and Emma still exist, even if Mary isn't consciously thinking about them. And then she (and the reader) can be surprised when Emma appears at her elbow... but see, the reader will then have the right experience, not "I must have missed something" but "oh, right, Peter must have carried her in without my noticing-- that's what that second door slam was."
No substitute for being in the character, for experiencing with the character. That's what you want the reader to do, right? So you have to do it first.