Every adventurer is a rogue.
By that, I mean that every adventurer is supposed to be good at all the things that rogues are specialized at doing. Brave knights in heavy armor are supposed to be sneaking up on sleeping dragons, treacherous wizard-apprentices are supposed to be stabbing people in the back with actual knives, and everyone is supposed to be finding and disarming traps and be fiddling with strange magic items to hilarious effect when they backfire. Any character in a fantasy adventure should be talking to NPCs, not just the Rogue who happens to be the only guy who had enough skill points to max out Diplomacy and ends up the only guy talking to NPCs.
That’s just what heroes in the source material do. Seriously, watch any fantasy movie or read any fantasy novel of the last 20 years and find me one that doesn’t do any sneaking around enemy bases, backstabbing dangerous beasts from behind, charming and conning princesses and pirate kings, activating strange and mysterious magic items, or avoiding some mechanical or magical traps.
In various editions of DnD, all of that belonged to the Rogue (of course, he used to be called the Thief, but I think everyone realized that most adventurers do a lot of thieving too). He was a clone of Fafhrd and the Gray Mouser and everyone assumed that they had those abilities because they were thieves and not because they were adventurers.
“Role-protection” is the idea that only some classes get certain kind of abilities, and there is some wisdom in that. No one wants to play a character at the table that someone else is also playing because the game gets a little less awesome when people are spamming the same abilities. That being said, DnD went overboard with the idea right at the very beginning of its inception and now “the Rogue is the only guy who gets to sneak around” is the biggest sacred cow in DnD behind the six-stat system, Magic Missiles, and using the most inconvenient forms of dice like the d20 that seems to have magic powers to roll off tables and under furniture.
Now, the last few editions tried to open that up a bit, but it never really worked. Cross-classing skills let some people get sneaking abilities and spellcasters have always been able to do everything the other classes could do, but at the end of the day the fact that only one or two guys at the table could use Invisibility or sneak abilities meant that the whole party never snuck up to anything. Every once in a while, someone would go on a solo mission to scout something and the rest of table would go play Smash Brothers while the stealth guy did a solo quest.
Just once in my life have I had a table that decided to fully sneak up the party. Our DM had decided that clerics didn’t exist in his version of Eberron and so the party was going to be forced to rest or use potions if they wanted healing.
Let me say, it was kind of awesome. We forced everyone at the table to take Hide and Move Silently, even the Dwarven Fighter with an Int of 8 who had to blow all of his skill points to cross-class. We had piles of potions that enhanced both kinds of checks, and used spells to cover other weaknesses.
Then we just smashed the published adventure (Red Hand of Doom). We snuck up on enemies, gathered intel, and basically acted like you’d imagine a group of Navy SEALS would approach adventuring and not the old Munchkin “bust down the door!”
Some people would say, “that was fun for you, but some people like to play differently.” To that, I can only answer, “but don’t you want the option to play like that?”
I mean, doesn’t everyone want the option to chat up evil sorceresses, spot elaborate traps, and slip into enemy fortresses wearing the clothes of an enemy soldier like every other hero in fantasy?
If you are interested in fantasy heroes who sneak around like champs, try The Dead City Gambit.