“I didn’t understand dating at all,” she said. “It was just playing catch up. Even out of college and into my 20s, I always was trying to fake being an adult.” In an hour’s conversation, this was the closest she ever came to acknowledging that pediatric cancer hadn’t been entirely a walk in the park.

Bayer didn’t mind lending Joanna her medical chart — she has never been shy about her diagnosis. As a teen she used it to win her family a trip to Hawaii courtesy of the Make-A-Wish Foundation. (She had thought about asking to meet Jared Leto, but she eventually decided she would prefer to meet him as a peer. Years later, she did.) Colleagues at “S.N.L.” have heard her introduce leukemia into the conversation just to get free ice cream, which jibes with advice she offered during our interview: If you are sick, use it to get whatever you can.

Bryant said, “She always takes the things that are hard and makes them something that she can use to empower herself or use to her advantage.”

Gradually, Joanna took shape, a woman more guarded than Bayer and more stunted, with her same love of snacks and her same gift for antic improvisation but none of her obvious success. A woman who lies about having cancer shouldn’t be a woman you root for, but Bayer has a way of communicating a kind of desperate brightness that makes terrible things seem less terrible, just because she does them with such enthusiasm.

What the camera recognizes is what Shannon, who also survived a major childhood trauma (her mother, youngest sister and a cousin were killed in a car accident), identified as a shared joy and determination to wring the utmost out of life.

“We don’t take it for granted,” Shannon said. “We feel so lucky that we’re alive. For real.”

There are many stories about illness. (Admittedly, there are fewer of them set in the world of home shopping.) But this is one — with its snacks and its sunniness and its heroine’s determination to exploit her fake diagnosis for all she can — that seemingly only Bayer can tell.

“I always wanted to do something about when I was sick,” she said contentedly, as the gentle chaos of Central Park swirled around her. “Specifically, the fun I had.”

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