The best actors have a quality that makes them seem more alive than the rest of us. It’s how they manage to embody other characters; it’s what makes them stars. So if the subtext of Valerie Harper’s appearance at the National Archives — and what drew so many fans to worship at her feet — was her terminal cancer diagnosis, it was easily forgotten. This woman is alive. And she is a talker!

“Well, look at you all!” the veteran sitcom superstar, 74, exclaimed, stepping onto the stage. “I know there are lots of places you could be other than my, what is it? A ‘colloquy’? Is that the word?”

And then Harper — a star of “The Mary Tyler Moore Show” and spin-off “Rhoda” — was off and running. Her appearance Tuesday night was a resumption of sorts of a book tour — for her memoir, “I, Rhoda” —  that was cut short by her brain cancer diagnosis in January.

“We were going to call it “I Rhoda Book,” but Simon & Schuster said ‘Oh, Val. . .’ Geez, I loved that title. But it was a serious book.”

She was chipper and engaged on the topic of her illness, sounding more like she was describing a really interesting medical documentary than a condition doctors say could kill her within months. “It’s cancer of the meninges. . . And they can’t do anything because it’s free-floating cancer cells in the [membrane] that’s spread over the brain. Like Saran Wrap.” Harper decided to go public with her diagnosis — “My husband said, ‘You’ll have control of the news'” — and since then has enjoyed a barrage of letters, flowers, medical advice from well-meaning strangers (“this heating mud, you need to try”).

Her attitude: “Don’t sit shiva before the funeral. Don’t spend your last days on earth thinking about your death. . . . People magazine called it “ready to say goodbye!” — but I’m also ready to say hello! I really am!”

Time for questions. How did “Rhoda” change her? “Oh, she made me wealthy! For a while! In showbiz, you’re up and down. We’re like farmworkers. ‘Oh the soybeans didn’t come in this year.'”

Her happiest day? “That’s tough because it’s all in the past. I appreciate it, I remember it — but today’s the best day.”

Biggest regret? “You know, I’ve tried to excise regret and remorse and replace it with responsibility.” And then off on a tangent about why you should never take your good reviews to heart, because then your bad reviews will destroy you, and how back in the day, New York had six newspapers reviewing her shows, “and three would love it and three would hate it — it was very egalitarian!” She stopped herself. “I got off your question, sorry.”

Can we find the later seasons of “Rhoda” on DVD? Not yet, but “maybe now with my diagnosis they’ll start releasing stuff. Hahahahaha!”

Harper said she just taped a spot on the Betty White sitcom “Hot in Cleveland,” that will serve as a mini reunion for the “MTM” gang, with fellow guest stars Moore, Cloris Leachman and Georgia Engel. “I think they did it because of the diagnosis,” she confided with a giggle. “They said, ‘Val seems fine, let’s get her before she croaks.’ Hahahahaha!”

Only once did Harper get choked up — as she recalled her 2010 Tony nomination (playing Tallulah Bankhead in “Looped”).

“Oh, gosh, it feels like a real accomplishment,” she said, her voice breaking. “Sorry for crying.”