It turns out Gulman was getting annoyed while doing his stand up in the scene and one person in the crowd kept laughing at the wrong time, throwing him off his game.
That person was Phoenix in character as Arthur Fleck.
In an interview with Collider, Gulman was about to talk to director Todd Phillips about the annoying extra when he realised who it was.
“The one thing I will say that was a little bit irritating is that when I was running through my set, there were all these extras, and they’d clearly been told that I was doing well, so they were laughing and enthusiastic,” he said.
“And there was one extra who was so enthusiastic in his laughter that it was throwing off my timing. He was just laughing too loud, and right when I was about to say something to Todd about maybe getting the guy not to laugh so loud, I realized that it was Joaquin Phoenix. It’s my best Joker story. I can’t believe it took me six takes to figure out that the man with the really bizarre laugh was the Joker.”
In the scene, Arthur is attending a comedy show at Pogo’s Comedy Club in Gotham City. The laughter that sends chills down our spines was heard during Gulman’s set, but not during typical laugh lines.
It’s unsurprising that Phoenix’s laugh would have thrown Gulman off so much, it is ridiculously creepy.
Gulman also revealed to the publication how he got the role, “I didn’t even know I was up for the role. The casting director was familiar with my work. I don’t know the whole story, but it seems that my agent made the casting director aware of my work, because they must have been looking for a comedian who was experienced, but not famous, because I think it would’ve thrown the people in the theater.”
“But I found I had it before I even knew it was a possibility, so it was really the best way, because I know that when you audition for things, you get disappointed, you worry, and in this case, I didn’t even know I was up for it.”
He added, “[Phillips] called me and asked, ‘what would you want to do for a joke? The only thing is, it can’t be something that’s not time-appropriate. You can’t be doing jokes about cell phones or the internet because it’s not of that period.’ So I suggested this joke about role-playing, where I’m a professor, and he said ‘okay, do that one.’
“So I went in a few days early to get fitted for a suit, because they made me a period suit, and then I went in. Other than talking over the scene with Todd a little bit, I didn’t really have to do much. They gave me two pages of the script, because they didn’t want the whole script out there.”