The "Girls Trip" star plays Crystal's sidekick in the film, about an ageing writer struggling with dementia, and she had a blast working alongside the comedy icon - until she was required to tap into her emotions on camera.
"The only thing I had an issue with was when Billy was asking me to cry," Haddish told U.S. breakfast show "Today".
The actress, who had a tough childhood growing up in various foster homes, said, "I was like, 'I've been spending 39 years of my life suppressing tears, not crying in front of people - do you know how hard it was to train myself to turn all tears into jokes?' "
However, Haddish was eventually able to get the waterworks going after a little coaxing from Crystal.
"(Billy) was like, 'Nah, I want you to cry.' And he got it out of me," she shared, explaining she "just stayed in the moment" and "listened."
"It was hard because I do it (cry) with my friends, in real life, but to do it with a whole crew around... I've trained myself to be like, 'When I feel like busting out with the Viola Davis snot tears, just make a joke! Do something, laugh, swallow those tears! Do something to make it go away.' "
"And Billy was like, 'Don't do it. Just be.' "
Despite having to face her fear of crying in public, Haddish insists the whole experience with Crystal was a dream come true.
Recalling her disbelief at being approached by his team to work together, she remembered, "My agent called me and was like, 'Billy Crystal would like to talk to you, he would like to meet with you.' I was like, 'Billy Crystal wants to talk to me? Are you serious?' "