By Lisa Respers France, CNN

The rave reviews are rolling in for Joaquin Phoenix in "Joker" and we are hoping someone, somewhere got behind-the-scenes footage they are willing to share.

The actor is known to be not only talented, but also a bit of a character himself.

Remember, he's a man who years ago had the public believing he actually had retired from acting to get into hip hop.

At any rate, his "Joker" director Todd Phillips has spilled some interesting tea about what life was like on set with Phoenix.

Phillips, who also co-wrote the film, told the New York Times that "there were moments when Phoenix lost his composure on the set of "Joker," sometimes to the bafflement of his co-stars."

"In the middle of the scene, he'll just walk away and walk out," Phillips said. "And the poor other actor thinks it's them and it was never them -- it was always him, and he just wasn't feeling it."

There was one person who apparently didn't see that side of Phoenix.

Robert De Niro, who has been known to get deeply into characters as well, said Phoenix was a total professional.

"Joaquin was very intense in what he was doing, as it should be, as he should be," De Niro said. "There's nothing to talk about, personally, on the side, 'Let's have coffee.' Let's just do the stuff."

Joker is set to be released October 4.