Originally Posted by Starbuck_II
That...doesn't really fit, in either a Watsonian or Doylist sense. Among other things:
Watsonian: Arthur's coworkers pretty clearly interact with Randall and Gary. Arthur tells the boss that Gary called him in, so either the boss is a delusion (which leads to a bigger set of problems) or the boss just accepts Arthur talking about a nonexistent Gary. The coworkers commiserate with Arthur at Randall's prompting. And so on.
Doylist: Arthur's delusions come from the things he wants. He wants a father and to be important - so he has a delusion of a situation where he gains a famous father figure. He wants both a girlfriend and someone who (unlike his mother) supports his creative hobby - so he has a delusion of a girlfriend that thinks his standup is funny. Also, his delusions aren't of dreamt-up people - it's of real people, just doing things in the way he wants.
Additionally, that way lies the idea that the entire movie is Arthur's delusion. Which...kinda ruins the movie, unless played off in the "multiple choice origin story" way.