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Nolan North voices some defective turrets and personality cores. One of them, a manly action-hero wannabe who calls himelf "Rick The Adventure Sphere", is a clear nod to Nathan Drake, perhaps his most well known voice role.
In the trailer for the Perpetual Testing Initiative, Cave Johnson starts his speech by talking about his father, who was a "professor of farming at the local farm college". JK Simmons plays Nathaniel Burke, a professor training insurance agents in the Farmers Insurance commercials.
"Portal Kombat": The release date coincided with that of Mortal Kombat 9's; the temptation to switch out the "M" in "Mortal" for a "P" proved to be too great. It doesn't help matters that both Noob Saibot and Scorpion have fatalities involving creative use of portals, and the former's babality even involves portals.
Developer Nickname: PotatOS, as seen in the developer's commentary and the official soundtrack, "PotatOS Lament".
Flip Flop of God: Valve just can't seem to decide whether Caroline is really deleted, or if she's Chell's biological mother. This is caused mainly by lead writer Erick Wolpaw and GLaDOS' voice actress Ellen McLain having different ideas on which should be true, and both having some degree of creative control over the storyline. Whether or not Caroline is really deleted is still up in the air, but there is more evidence to imply that Caroline is not Chell's biological mother, as Caroline would've hit menopause long before Chell was born. Of course, that might not stop SCIENCE!
Two clipboards from the first game are reused without any change in design. One of them is the "Hazard Identification Card" which shows the icons for the first game's test chamber banners. This means that the clipboard's icons for the High-Energy Pellet and cake remain in the second game even though the Nostalgia Levels' test chamber banners have replaced the corresponding icons. The other clipboard is the Test Subject #234 clipboard located near the area where turrets are checked for defects.
The Radios that play 'Still Alive' show up again in at least two spots. When you first re-enter the ruins of the testing facility, you'll find one and when you get to the level with the garbage thrown out by GlaDOS, one of the garbage items is a radio and you can take it to Rattman's lair on the same map to get an achievement.
Throw It In: Both J. K. Simmons and Stephen Merchant improvised many of their lines. Contrast Ellen McLain, who stuck very close to the script (she consistently describes herself as an "interpretive" artist, as opposed to a "creative" artist, even though she wrote the lyrics for the Turret Opera and PotatOS' Lament).
Urban Legend of Zelda: Originally, there was meant to be an additional scene showing Caroline being forced into the mind uploading despite her objections, but the scene was later cut. Since a few of Ellen McLain's lines in the scene were left in the game files, numerous rumors began to circulate that either A) the scene was initially present in the plot but later removed; or B) since the scene played out like a rape, several cast and crew members objected to it (including Ellen McLain, who burst into tears, and J.K. Simmons, who refused to record lines for the scene, hence the absence of his lines in the game files), which ultimately forced it to be cut. Despite the obvious contradiction of J.K. Simmons getting offended at acting in a rape scene despite doing an actual rape scene previously in Oz, Erik Wolpaw has had to confirm that these rumors are false. No comments have been made on the actual reason for the cutting of the scene.
A very early version of the game, based on an internal prototype called F-Stop, was supposed to be a full prequel to Portal, taking place decades before the main game and focusing on Aperture's origins, with a brand-new game mechanic that Valve absolutely loved - and without a single portal in sight. Testers, on the other hand, weren't so happy about the concept, insisting on having Chell, GlaDOS and the Portal Gun in the sequel. Obviously, Cave Johnson's mishaps in The Fifties were incorporated into the abandoned test areas.
Valve released short Aperture Science videos showcasing the various new gimmicks, including Aerial Faith Plates, the Gels, the Thermal Discouragement Beams, and the Pneumatic Diversity Vents. The last is completely absent in the final game. Although the player does travel through vents at several points, they're cutscenes, not a gameplay element.
Originally there were meant to be more personality cores the player would interact with apart from Wheatley. The audio commentary explains they scrapped them because there were too many, and there wasn't enough time to give each one a decent amount of characterization. Some of the cores got recycled as the cores you use to corrupt Wheatley during the final battle - notably, the Space Core, the Adventure Core, and the Fact Core.
Some unused dialogue found in the Portal 2 files suggests that Cave Johnson didn't die before being uploaded into a computer much like his Memo◊ suggests. In fact he would have been stored in a cube similar to the companion cube and would be plugged into a wall. In order to get through to the next chamber you would need to disconnect and kill this cube In fact Cave begs for death saying "My life is torture, please kill me." You would also have to climb upon his dead/dying corpse to get to the next chamber.
If Stephen Merchant turned to be unavailable, Valve were in talks to have Richard Ayoade - best known as Moss from The IT Crowd and Dean Learner/Thorton Reed from Garth Marenghis Darkplace - play Wheatley instead.
The co-op mode was originally going to have another woman named Mel work alongside of Chell. She ended up being replaced by Atlas and P-body after someone pointed out that play testers tended to die/screw each other over a lot and having robots in that situation would make it hilarious instead of horrific.
One of the gels was supposed to allow you to ignore gravity and walk up walls. It made playtesters sick. It was one of the three "paints" in the indie game "TAG - The Power of Paint", that inspired the gel mechanic. The other two, "Fast" and "Jump/Bounce", made their way in the game with the new Conversion Gel.
A fan named Omnicoder rebuilt it from scratch, but never released his work. Another fan named Felix Griffin rebuilt it separately several years later.
One player found that the Peer Review DLC secretly replaced the unused fourth gel with a new "Reflection Gel", which would reflect any lasers directed on to them similarly to a Redirection Cube. It was most likely cut because the cubes are far more versatile and flexible than the gel when it comes to devising puzzle solutions.
Cut sound clips seem to indicate that the original plan was not to have a separate announcer narrate the tests before GLaDOS's revival, but recordings of GLaDOS with the exact same lines. Notably, these lines bear a stronger resemblance to her lines from early on in the first game (i.e. more cold and robotic) than the rest of her lines, suggesting a sort of audio continuity.
According to The Final Hours of Portal 2, there were several different ideas for the ending, including one where Chell would say one word, "yes", to end the game, and another involving a duet between Chell and GLaDOS.
One idea to explain why GLaDOS wasn't helping Chell solve the test chambers in Chapter 8 was that the bird would keep returning to peck out bits of her, removing her memories of how to solve them and eventually making her as dumb as Wheatley. Various production issues made it impossible to implement.
Some Dummied Out lines of dialogue suggests that Wheatley was the one who released the neurotoxin on "Bring your Daughter to Work Day", rather than GLaDOS.
Wheatley: Over there is where they used to keep the old neurotoxin release button. BIG responsibility, the guy in charge of the neurotoxin release button. And guess who he WAS? He wasn't me. But I was his assistant, and I did a lot of his admin. You know, in the end, yes, they let me go. It's all politics, to be honest. It's a big popularity contest, it's all about who you know, and whose back you're willing to scratch, who doesn't touch — or, in my case, who did accidentally touch — the neurotoxin button. But not entirely my fault! You shoulda seen the SIZE of that thing, it was huge! I should have gotten a raise for all the times I DIDN'T bump into it."