Selma Blair could only talk for a half-hour in our first session. That was as long as she trusted her brain and her body to cooperate — any longer and she feared that her focus might start to wander or her speech might begin to trail. “We’re being responsible in knowing that smaller moments will be clearer moments,” she said.For Blair no day is free from the effects of multiple sclerosis, the autoimmune disease that she learned she had in 2018 but that she believes began attacking her central nervous system many years earlier.This particular Friday in September had started out especially tough: She said she woke up in her Los Angeles home feeling “just bad as all get out,” but she found that talking with people helped alleviate her discomfort. Blair said she had had good conversations earlier in the day and that she had been looking forward to ours.So, if she needed to take a break during this interview, she said with a delighted cackle, “it just means you’re boring me.”An unparalleled lack of inhibition has always defined Blair’s best-known work. She is 49 now, with a résumé that includes seminal works of teensploitation (“Cruel Intentions”), comedy (“Legally Blonde”) and comic-book adventure (“Hellboy”).Blair in one of her signature roles, as a fellow law student opposite Reese Witherspoon in “Legally Blonde.”Tracy Bennett/MGMThat same unbridled bluntness persists in all her interactions, whether scripted or spontaneous, with cameras on or off, even when she is sharing her account of the time she went on “The Tonight Show” wearing a strappy top she accidentally put on sideways. It is a story she told me proudly, within five minutes of our introduction on a video call, while her fingers made a maelstrom of her close-cropped, bleached-blond hair. (By way of explaining this style choice, she burst into a brassy, Ethel Merman-esque voice and sang, “I want to be a shiksa.”)But Blair’s candor has come to mean something more in the three years since she went public about her M.S. diagnosis. Now, whether she is posting personal diaries on social media or appearing on a red carpet, she understands she is a representative with an opportunity to educate a wider audience about what she and others with M.S. are experiencing.It is a philosophy of maximum openness that she is taking further by appearing as the subject of a new documentary, “Introducing, Selma Blair.” The film, directed by Rachel Fleit, is an unflinching account of Blair’s life with M.S. and the stem-cell transplant she underwent to treat it in 2019. (The documentary will be released in theaters on Oct. 15 and will begin streaming Oct. 21 on Discovery+.)As Blair explained, she was hopeful that the film would be meaningful to viewers who feel challenged and uncertain, whether or not they have a chronic illness.“This is my human condition,” she said, “and everyone has their own, but I think we are united in feeling alone or frightened when we have a big change in our lives. This wasn’t a vanity project at all, and I’m very capable of loving vanity.”Blair said that since she went public with her diagnosis, she’s been offered roles for aging or disabled characters: “I might be those things, but I’m still everything else I was before, and I shouldn’t be relegated to that.”Magdalena Wosinska for The New York TimesFor Blair, the documentary is just one piece of a larger effort to understand herself — to determine how much of her identity has been shaped by her disease, and what will remain or change now that she is being treated for it.“If this had happened in my 20s, when I’m trying to start a career and set a few shekels aside, I would have been mortified,” she said. “I’m old enough now. I’m getting to know a whole different personality, and I’m not ashamed.”Thinking back to her upbringing in suburban Michigan, Blair described herself as a 7-year-old who toted around her own copy of the Physicians’ Desk Reference, the massive tome of information on prescription drugs, and wondered why she experienced constant pain, fatigue and unpredictable mood swings.These difficulties persisted into adulthood: The pain got worse, particularly after the birth of her son, Arthur, in 2011; she had problems with her vision and experienced involuntary muscle contractions in her neck.Until she received her diagnosis, Blair said, she couldn’t understand why her symptoms varied from setting to setting. “I can walk better in my house, but outside it’s like a sand pit,” she said. “With certain light, my speech becomes intermittent even though my larynx is fine.”“It never occurred to me that there’s a traffic jam that happens in my brain,” she said.In the flurry of attention that followed Blair’s disclosure of her diagnosis, she was introduced to Fleit, and they agreed to start shooting the documentary in the days just before Blair traveled to Chicago for her stem-cell transplant.Fleit said that Blair exercised no editorial control over the film, adding that the endeavor would succeed only if the actress “was willing to show the world what really happened — that brutal intimacy and honesty that you just don’t see — and she was totally open to that.”Fleit, who has alopecia universalis, an autoimmune disease that causes hair loss, said she felt a particular connection to Blair as filming proceeded.“Being a bald lady in the world has given me unique access to a certain kind of emotional pain,” Fleit said. “It does not frighten me anymore, and I feel uniquely qualified to hold the space for another person who’s experiencing that.”But not everyone in Blair’s life was immediately comfortable with her pursuing both the film and the stem cell transplant. Sarah Michelle Gellar, Blair’s “Cruel Intentions” co-star and longtime friend, said that she was fearful about the treatment, which was accompanied by an intensive chemotherapy regimen.“I just felt like it was so risky,” Gellar said. “And her attitude was, yes, I’m managing right now, but in 10 years I might not be, and I won’t be a candidate for this treatment. It was now or never. And now or never is a very good definition of Selma.”The actress with her son, Arthur, in a scene from “Introducing, Selma Blair.”Discovery+Gellar was also unsure about the film project — “I’m a very private person, I can barely share going to the supermarket,” she said — but she understood Blair’s position: She felt it was important for her son.As Gellar recalled, “She would say, ‘God forbid, if I don’t make it, then Arthur has a whole video diary of what I went through. He’ll never have to wonder, did I give up? He’ll know how hard I fought to be there for him.’”To Parker Posey, a friend and colleague of Blair’s for nearly 20 years, the decision to make a documentary was as much a legitimate form of expression as any other artistic enterprise.“This is the only thing we have — your life as an actor, it’s all material, it’s all story,” Posey said. “Am I going to land in something that gives me meaning, away from the pettiness of most entertainment?”Posey added, “Anyone who can find purpose in creating what they’re supposed to create and bravely live their life, that’s art. That’s the triumph.”Blair, for her part, said that once shooting started on the documentary, “I don’t think I noticed. There was really no directing and I mean that in the best way.”She added, “I don’t think I’ve realized that a film is coming out where I’m the subject of it. I haven’t really processed that.”With our half-hour coming to its end, we said our goodbyes and I told Blair I looked forward to reconnecting with her in a few days. In a comically ethereal voice, she answered, “God willing, if I’m alive.”Our next session, planned for that Monday, had to be delayed when Blair fell from a horse she was riding over the weekend. As she told me in a follow-up conversation — this time over the phone, as video calls were making it difficult for her to focus — she had lost her balance and hyperextended her thumb but was otherwise doing OK.She was more embarrassed by how she felt she’d behaved in our first conversation, using her admittedly outrageous sense of humor to paper over her anxiety. “I get so spooked because there is still, even in my mind, a stigma of, you won’t bring it — you won’t be able to make this mind-body thing work,” she said. “I’ll use the defense of a shtick when I feel like I’m faltering.”She was also bothered by a remark she had seen on her Instagram account from someone who offered support for her documentary but said, as Blair described the comment, “I wish a regular person were doing it, like a person that’s not a celebrity, because it’s not the same.”Blair emphatically added, “I am a regular person.”“This wasn’t a vanity project at all, and I’m very capable of loving vanity,” Blair said.Magdalena Wosinska for The New York TimesCynthia Zagieboylo, the president and chief executive of the National Multiple Sclerosis Society, said that Blair’s decision to share the story of her experience could be beneficial to other people who have the disease and those who want to know more about it.“There isn’t a right way to move through something like this,” Zagieboylo said. “There are no two stories of M.S. that are the same and for people to express themselves, it’s very personal.”When someone like Blair is open about her illness, Zagieboylo said, “people can feel less alone in facing the challenges of their own M.S. People experiencing potential symptoms might recognize something. It could lead to an earlier confirmed diagnosis of M.S., which means people could get treated faster and that leads to better outcomes.”She added, “By her sharing her journey with the world in a really authentic way, there’s really no downside to that.”Blair said that she had been told her M.S. was in remission, which she said meant “there is not a clear path for my disease to get worse, and that’s huge. That gives you breathing room.” There was no certain timetable for how long her stem-cell transplant might be effective but, as she said in her characteristic style, “I could get hit by a bus before that.”One of the strange benefits of this period of relative calm is the chance to learn whether past behaviors that she considered fundamental components of her mood and personality — the outbursts, the impulsivity — might be manifestations of her disease.Blair described a conversation with a neurologist who asked if she took medication for pseudobulbar affect, a condition that can result in sudden uncontrollable laughing, crying or anger.“I said, ‘No, this is just me, what are you talking about?’” Blair recalled. “She’s like, ‘Or maybe it’s not.’ It never occurred to me.”Blair added, “I don’t know if I will ever work my way out of neurological damage. I know I can find new pathways, but I’ve been scarred for so long.”Sarah Michelle Gellar said that she and other friends of Blair were concerned about the film project and the stem cell treatment, but that Blair had reassured them. Magdalena Wosinska for The New York TimesShe continues to help raise Arthur, whose custody she shares with his father, Jason Bleick, a fashion designer and her former boyfriend. But she said her son had not been able to watch all of the documentary.“About 20 minutes in, he wasn’t comfortable,” she said. “He was worried that people would see me this way and talk behind my back or not give me a job.”Blair said she very much intended to keep working as an actress and, to whatever extent she’s perceived as having stepped back from the industry, it’s not because she isn’t putting herself out there for roles.“The parts that I’m offered since I’ve had my diagnosis are the old woman, the person in the wheelchair, the person bumping into walls,” Blair said. “I might be those things, but I’m still everything else I was before, and I shouldn’t be relegated to that.”But now that she has put herself out there in the truest way she knows how, Blair hopes that her efforts will remind others — and reinforce in herself — that there is value in this kind of transparency.“There’s a difference it can make to people,” she said. “I don’t mean it in a flaky, soft way. I mean, really make the time to go beyond, because you never know what people are holding inside, and what a relief to know even adorable people like me” — she could not suppress one last knowing laugh — “are troubled by their own brains and bodies at times. That’s the comfort I wish I could give.”