The 49-year-old actor reprises the role of Arthur Fleck/The Joker in the sequel and reflected on how he received vocal tips from the pop star turned actress - who stars as music therapist Lee/ Harley Quinn.
In an interview with Empire magazine, Joaquin quipped: "I do seem to remember her spitting up coffee the first time I sang, so that felt good, that was exciting, and made me feel confident".
"Gaga was always very encouraging of just, 'Go with what you feel, it's fine'".
He added: "For somebody who's not a performer in that way, it can be... uncomfortable to do that, but also very exciting".
Joaquin also explained that his mentally ill character's singing voice was intentionally imperfect.
The Oscar-winning star said: "It was important to protect that with poor phrasing and occasional bum notes".
"Arthur grew up hearing his mother play these songs on the radio. He's not a singer, and he shouldn't sound like a professional singer. He should sound like somebody that's taking a shower and just bursts out into song".
Gaga explained that she will be showcasing a different singing voice from the norm through her part in the picture.
The 'Poker Face' hitmaker said: "People know me by my stage name, Lady Gaga, right? That's me as that performer, but that is not what this movie is; I'm playing a character".
"So I worked a lot on the way that I sang to come from Lee, and to not come from me as a performer".