Call of Duty 4: Modern Warfare is a game filled with memorable and explosive set pieces, but the central idea behind what would become one of the franchise's most iconic missions, All Ghillied Up, didn't initially win over many developers at Infinity Ward.

The mission is now mimicked in nearly every modern military shooter, including almost every Call of Duty to come after 2007's Modern Warfare. It's an idea that is now iconic and almost synonymous with the franchise: two operatives, deep behind enemy lines wearing ghillie suits that make them almost invisible to the naked eye, equipped with suppressed sniper rifles. One wrong move could spell certain doom, and so players are forced to take their time, take strategic shots, avoid enemies, and work together with their NPC partner to survive.

In an interview with IGN to mark the franchise's 20th anniversary, one of the key minds behind All Ghillied Up, Modern Warfare designer Mohammad Alavi, said the idea of making Call of Duty's first real stealth mission wasn't a popular one among the team.

"Nobody thought it was cool, including myself," Alavi said. "I always like the more bombastic missions and this was the opposite of that."

Thankfully, other Infinity Ward members like Steve Fukuda, one of the game's lead designers, and Preston Glenn, a level designer, were able to see the potential of a stealth-based mission, eventually winning over Alavi.

"[Fukuda]'s like, 'You're walking in a field, you come up to a bush and you see two guys past the bush and you raise your gun,'" Alavi said. "'You're about to take a shot, and the bush turns to you and goes, hold up.' And that's all he said. And I was like, 'I'm in.'"

Crafting All Ghillied Up wasn't easy, however. Alavi said that making the mission essentially meant reworking how Call of Duty's code worked.

"It was war all the time," Alavi said. "The AI was never designed to not see you. So I was like, 'We can fake this.' But I don't like doing that because you can see through it instantly, right? I was like 'Or, I can make this Metal Gear Solid style. I can make this the best stealth mission in a Call of Duty game.' And without telling anybody, because I knew they were going to try to stop me, that's what I tried to do."

The rest, as they say, is history. Alavi had to rework the ability for enemies to see players and write "almost 20,000 lines of poorly written script" to custom-make the ability for players to hide in shadows and bushes undetected.

Be sure to head over to IGN to read the full story on how All Ghillied Up came to be. One of Call of Duty's original creators also recently shared how the franchise started out as trying to outdo Counter-Strike, but only came into its own when it cast off that aspiration and instead embraced the idea of making Team Deathmatch as much fun as it could be.