"The first film really deeply moved me, I loved it so much," Lady Gaga told a press conference, calling Phoenix's performance one that "set the bar extremely high."
That idea took flight, and Lady Gaga was the next piece of the puzzle, Todd Phillips said.
And ultimately we did it and it was really the only way," Phoenix told journalists.
The film's music was a vehicle to "give characters way to express what they need to say," Lady Gaga said.
Lady Gaga later announced on Instagram that the first single on her next album called LG7 would be released in October.

Venice Film Festival: Joaquin Phoenix, Lady Gaga, Todd Phillips At Joker: Folie a Deux Premiere