What some call "discipline," Joan Rivers calls "desperation"... half-jokingly, of course. The legendary comedian spoke to HitFix and to enthusiastic audiences after a screening of her doc "Joan Rivers: A Piece of Work" at Sundance on Tuesday, and there's a lot to address -- including that desperation to work hard in a comedy world that seemingly only retorts to her past and present enterprise with cracks on plastic surgery and her age.

The film follows the 75-year-old performer for a year of trials and triumphs -- her winning stint on NBC's "Celebrity Apprentice," empty calendar dates, the rise and fall of her new play, crash-testing jokes on national tragedies and vaginas and even a short battle with an audience member in Wisconsin that angrily protests a joke about deaf children and Helen Keller by shouting at Rivers while she's on stage.

The latter sort of incident happens “maybe once in every six years,” Rivers tells HitFix. “I felt terribly sorry for the audience. They don’t know what to do. And you gotta bring 'em back and make them forget it, and make them realize that we’re gonna go right back and have a good time some more. Very difficult, very difficult.”

The key to Rivers’ humor, and the key to her own healing and happiness, is to laugh at one’s self, to take hardship less seriously. “If you laugh at it, you can deal with it,” she continues. “My mother couldn’t see toward the end [of her life], and I lived with a man for nine years that had lost a leg in WWII… and see now I love handicap bathrooms. If your plane is delayed, you can lock yourself in there, do some yoga. And if you hear a knock on the door, ‘I need to get in’… too bad, roll somewhere else.”

Her gift of comedy came in handy when her late-night television show was canceled in the ‘80s, or when her husband committed suicide. Or when her mentor and earliest cheerleader Johnny Carson quit talking to her, or when she was roasted on Comedy Central.

Each caused her pain, too. Rivers choked up in front of hundreds of audience members at Sundance (video below) as she talked on Carson’s abandoning her as a friend when she decided to move to FOX for her own show, after years of holding down the guest host spot on his “Tonight Show.”

In the interview, Rivers further explained “it was very painful. [Carson] was thrilled with the men that went on… Cosby went on, David Brenner, everybody went on and on. We all started in the Tonight Show. And everyone went with his blessing. And I was the one he thought he owned. Someone told me that he liked me ‘more than the others, so it hurt him more.’”

And in the film, she openly discussed how it hurt her feelings that the main target at her Comedy Central roast was her looks and her years.

“So with the roast, what I was worried about is what I was gonna say back to them. So I worked very hard. And I won,” she smiled during the interview, wine in hand. “For every comedian, if you’re not insecure you’re not funny. If you’re happy, you’re not funny.

She couldn't help roll her eyes and feign fainting as she discussed the opening sequence of "Piece of Work": a series of up-close shots of Rivers' bare face,  as she begins to apply her omnipresent cake makeup. "Once you commit to a documentary, with documentarians, then you must do what they wanna do. I gave them the reins. What you see is the truth."

Rivers plans “never ever ever ever ever ever” to retire and is currently at work on her third book and a new reality show with daughter Melissa, "Mother Knows Best." And she doesn’t appreciate up-and-comings that say she’s paved the way for them, as if the job’s done.

“I’m not dead yet, I’m not finished. Watch what I’m doing next. Don’t tell me I opened the door for you. I’ve got six more doors I’m opening and if you’re smart, get behind me. I’m not going to let you in so easily.”

Thanks to IndieWire for the YouTube footage.