If you continue to defy the Narrator in the HD remake, you'll come across two doors, red and blue, and the Narrator instructs Stanley to enter the red door. If you notice, next to the red door is a red hand truck—visual symbolism that by going in the red door, you're going back to being "carted around" by him.
The same red hand truck is right next to the cargo lift as well... Implying that you are going to get played with, should you ride it all the way to the phone.
And right next to the blue door, there is a blue paint can, a symbolism for art and creativity, maybe?
One of the only consistent descriptions that the Narrator gives Stanley is that his job is pushing buttons, and that is essentially the only thing that you can do as Stanley, you push buttons to go up or down lifts, you push buttons to open doors, you push buttons to give feedback, to turn off machines, and to save babies from fires. There is only one real instance in which you can make a choice which doesn't involve pushing a button, and that's when you pull the telephone plug out, at which point the Narrator realises that you can't be Stanley and instead must be a real person.
Naturally, you do that by pointing Stanley at the plug and pushing your action button.
You also move Stanley with buttons, unless you're using a joystick.
The number "8" is a number that is repeated often, such as in the Demo and, in the Real Person ending, the presentation claims that doctors recommend making 8 choices a day. Why Eight? Because eight is the Infinity Symbol on its side, and it has no beginning or end. The theme that the game has no ending is very prominent in several of the endings (Such as the Museum ending when the second Narrator says the two will be in conflict forever because the game restarts) and on the loading screen the text "THE END IS NEVER THE END IS NEVER THE END etc." The game and Stanley's manipulation by the Narrator is a never ending cycle, thus why the most important number is the one without beginning or end. 8.
However, on the Reddit AMA, Word of God states that they chose the number 8 because "Why not? Do you have a better number?"
When you step out into the wilderness in the Freedom ending, you lose control of Stanley and he starts moving on his own — the only instance when he does so. (In the Real Person ending, you also lose control of Stanley, but then he just stands forever in front of the two doors.) This makes sense: the Narrator tells that Stanley has broken free of control — including your, the player's, control.
In the Freedom ending, the Narrator mentions that Stanley doesn't want control, or knowledge. Just happiness. In the Countdown ending, the Narrator mocks Stanley for wanting control, then tells him where his coworkers went; AKA knowledge.
Also, did you notice that the Freedom ending, despite implying Stanley is free, is the only ending where you blindly follow all of the Narrator's commands? Could also be interpreted as Fridge Horror due to the feeling of not being able to escape no matter what.
From the HD Remix Demo: in the emotion booth room, there's only two booths you can enter, and most of the others are switched off. Behind the two open booths, in the back row, there's a single booth that's labelled and lit: Fear.
Mariella's reaction to encountering Stanley, dead of a mysterious brain dysfunction, is a fate-tempting self-satisfaction that she is normal and sane. Then much is made of her imminent job interview "with very important people whose decisions would impact her career, and, by extension, the rest of her life." It may be implied that Mariella is fated to be employed by Stanley's company, and has no idea that she is looking at her own likely future.
In fact, Mariella's mere existence in Stanley's world (the game), whether she is an NPC or a player/narrator-controlled character means that she is damned to the exact same fate as him whether she gets the job or not. She is no more a real person than the mannequin posing as Stanley's wife - the only reason she even exists is for this scene where she finds Stanley's body. The only difference between her and the dead Stanley is that she's unaware of this.
The Narrator seems at first to have total control over the story. He opens doors, locks off doors, shepherds Stanley through areas and even maliciously punishes him for not doing as he's told. But when the Narrator himself tries to deviate to any degree from the rough script (such as when he teams up with Stanley to make a new story), he's unable to do so and reality begins to collapse. So what unseen force is holding all the narrative power here? Not only is the Narrator dependent on Stanley, he doesn't even have the power to change the game and is just as much a slave to the unseen power as Stanley.
If nobody besides Stanley showed up in the building, then who let him inside in the first place? It's highly doubtful that they would give keys to the entire building to a lowly office worker, or that they would just leave the building open for the night.
Some places have automatic doors that open with a lanyard - it could have been that he was able to get in with his emplyee ID. Either that, or everyone else was spirited away after Stanley got in.
Or it's all in his head like the Narrator says in one ending.
The very simple answer (and the creepiest) is that Stanley didn't have to be let into the office because he came into existence in the office and doesn't exist outside the office. With the exception of the ending where the Narrator said he was found dead in a street, which is something the Narrator invented. Stanley's wife, kids etc are fake backstory and only ever described by the Narrator. He presses buttons "as if it was a job he was made for" because it is the job he was made for. He only exists for as long as you boot the videogame and play it - and his co-workers never even existed.