He's already started rapping. This is what he said while on stage.
“This is me saying this is who I am. This is my story. After all the years of reading scripts and reading lines, this is my chance to do something straight from the heart and put it out there.
“When I was young I liked punk rock music but then I discovered rap. I love the storytelling aspect of hip-hop.
“Are there people out there who think I’m a joke? I’m sure there will be. Are there people who think it’s going to suck? Probably, but I can’t worry about that.”
The way I see it is the only way that Ben Stiller did that parody while not thinking Joaquin Phoenix is serious is if he is in on the joke.