Players don’t tell the GM what their PC does, they say what their PC tries. The dice say what the PC actually does.
You can see this clearly demonstrated in the combat rules. Alric might set out to plant his axe in the forehead of some overwrought ogre, but if Alric’s player rolls an 18, it’s the Critical Miss Table that says where that axe really ends up. It happens outside of combat, too. Will rolls, especially, get to take the controls away from the player from time to time. If someone’s interrogating Needles, and he crit fails his Will roll, he’s going to let something spill, no matter what his player claims.
The dice also take care of everything that’s below the game’s level of detail. An example from the Space Cowboys game: Dr Nanika had just completed an examination of an unconscious patient, and hadn’t been able to determine a diagnosis. (I can’t say for certain, but I believe this was during the part of the story when they were trying to understand all the strange things that were happening due to the Zombie Plague, before they figured out there was such a thing.) In the real world, of course, this was no more than the player saying “I’ll examine him and try to figure out what’s up”, then rolling Diagnosis. In-game, Dr Nanika did her thing; just like an episode of any medical drama you care to name, she did tests and performed procedures and consulted references. After all was said and done, she came out of sickbay and announced that she was stumped. Bubba asks, “Did you check him for head wounds?”
The players turned to look at me expectantly. Nanika’s player half-reached for her dice. I pointed out that if Dr Nanika had forgotten to check for the possibility that the patient had been bopped on the head and knocked out, until the ship’s mechanic brought it up, she needed to go back to Mars University and demand a refund.
There’s a lot of games that my group could never play. We’re just not equipped. No matter how much we might enjoy watching the antics of TV doctors through the ages, we can’t talk that talk. Anything medical in our games is always going to come down to dice-rolling and hand-waving. I’m more likely to give a re-roll for the player chewing the scenery than I am for someone remembering an obscure medical factoid. (Doc throws her stethoscope to the side, rips Bubba’s shirt open, and starts beating him with a lead pipe while screaming, “Live, damn you, live! I won’t have another brother’s blood on my hands!”… yeah, that’s worth another throw of the dice.) I don’t have the knowledge to talk details, so the dice handle all that for me. Did the doc stitch the patient up correctly? Must have, since the roll for the procedure was a success…
What I do, as the GM, is, I’ll look at the dice, and base actions and outcomes on margin of success or failure. If the roll fails by just a tiny bit, one or two, then I’ll often describe it as a bit of bad luck, or something unexpected. The wind shifts, or a cloud goes across the sun. The sneaky person suddenly has to sneeze. If it’s an extravagant failure, by 8 or 9, I’ll aim more for incompetence, as the task is just beyond the character’s skills. Instead of cutting the red wire, the character slips and nicks the blue wire. The sneaky person trips over their own feet and falls out from behind the curtain. In the same way, if someone rolls really well, I’ll try to describe the action so as to make them look that much better, cooler, more skillful. That’s how we get things like Needles knowing how to disable the trap by grabbing it’s trigger just as it’s going off.