The 45-year-old actor won the award for Best Performance by an Actor in a Motion Picture -- Drama at the 77th annual awards show on Sunday, and was soon after brought into the press room to answer reporters' questions about his big win.
Unfortunately for those reporters, however, Phoenix wasn't too pleased with this step in the process, declaring that he had been "tricked" into the room by someone who told him it was just for photos. His dissatisfaction continued when a reporter asked him about the process of preparing for his role in Joker.
"Isn't this old news?" Phoenix snapped back. "I think I've talked about this for six months. I mean, right? I feel like I answered these questions. You want to hear a different version of it? I can try to change it slightly, or what should I do?"
The actor eventually gave in, discussing his "long process," which involved research into medications and political assassins, and later weight loss. "And there's probably so many other things that I just can't remember right now. But I feel like I already answered this question six months ago, but thanks for still asking," he quipped.
The next question came from another reporter asking about speculation that a Joker sequel was in the works -- and asked Phoenix to respond to criticism that a sequel would undermine the artistic expression of the film.
"What do you mean?" the actor shot back. "Why?"
Further explanation ended with the reporter clarifying the concerns didn't stem from him directly. "I don't care who is saying that," Phoenix said, before taking his response in another direction. "That's awesome and I love whoever says it."
"I don't think that I've ever done too many predictable things. So if that came up, it's not because we're following some rule book, it's because we feel inspired again to explore the character more thoroughly. That would be the only reason for me to do it. So that's all I can say about that," he added.
Phoenix then quizzed reporters, asking whether they were "bored" and what food they had before declaring that "there's more space in here than there was in there."
A few laughs from the crowd seemed to loosen Phoenix up, just in time for a final question about the awards ceremony's plant-based menu. Phoenix, a longtime vegan, lit up.
"It's the first time I've ever eaten the food at this ceremony. I thought it was excellent. I was so moved by their decision to make tonight plant-based. It was such an important step. The SAG Awards and Critics Choice Awards and Governor's Ball, whatever else there is, need to do it as well," he declared.
"I think we're coming to a point now where the evidence is irrefutable and undeniable. I think it's somewhat where we realized how poisonous tobacco was and secondhand smoking. They made it illegal to smoke in public places. I think now consuming animal products is no longer just a personal choice. It is having a drastic and vast consequence on the rest of the world and all of us, and so I think it was incredibly brave and compassionate of HFPA to make that bold decision. I really commend them for that," he continued. "I've never been so proud to attend an awards ceremony as I am tonight, and thank you so much for asking that question."
And with that, Phoenix wrapped up. "My publicist is telling me I need to go home," he confessed. "Thank you very much."
The actor's awkward press room appearance came soon after his bizarre acceptance speech in the Golden Globes ballroom, during which he joked that he felt intimidated by other actors even though they "shared the same agent."
See more on Phoenix in the video below.