Seated on a stool, Angela Lansbury addresses the camera as a pistol goes off.

A bullet travels toward her in menacing slow motion, but she seems unaware. She is talking about amyotrophic lateral sclerosis (ALS) — Lou Gehrig's disease — which progressively paralyzes its victims and cruelly shortens their lives.

Then Lansbury reassures her audience that continued funding for research “will help people with ALS do this” — she rises to her feet and pertly steps out of view, having dodged the bullet by a fraction of an inch as it pierces the wall behind her.

This forceful TV spot is part of a public-awareness campaign featuring Lansbury as the new spokeswoman for the ALS Association.

“We use a bullet as a metaphor,” says Lansbury, “and it's a very apt one. With ALS, you don't really know where it comes from, and you don't know when it might hit.”

In short, much mystery enshrouds this disease, which, according to the ALS Association, afflicts some 30,000 Americans, and so far has eluded efforts to uncover any clear-cut cause.

Lansbury's sister died from ALS two decades ago.

“It's a terrible disease,” she says. “I wanted to get on board and help to make the American public aware how desperate the need for research is.”

Lansbury makes a fitting advocate. She remains widely known from her dozen seasons on the CBS whodunit “Murder, She Wrote,” which premiered a quarter-century ago. She played a crime novelist cracking mysteries with lovable gusto.

An image like that “does help. People trust me,” she says, then chuckles, “They shouldn't, really. I'm NOT Jessica Fletcher. But I share a lot of her qualities, I hope: I'm interested; I care.”

Of course, the London-born Lansbury has far more credits than this hit TV series. At 82, she's an actress whose range and longevity qualify her as a full-of-life example of show-biz history.

For her Oscar-nominated debut in 1944, she joined Ingrid Bergman and Charles Boyer in the suspenseful classic “Gaslight.” Soon after, she played Elizabeth Taylor's big sister in “National Velvet.”

Never an ingenue or leading lady, Lansbury typically played take-charge types, whether the ribald flapper of Broadway's eponymous “Mame” or Laurence Harvey's power-mad mom in the 1962 thriller “The Manchurian Candidate,” or even kindly Mrs. Potts, the tea pot she voiced in the 1991 cartoon “Beauty and the Beast.”

Winner of four Tonys, she most recently appeared on Broadway just a year ago, winning raves in Terrence McNally's comedy “Deuce.”

Now, when Lansbury pauses long enough to take stock, “I'm astonished,” she concedes, “at the amount of stuff I managed to pack into the years that I have been in the business. And I'm still here!”

Granted, she has no acting projects currently lined up.

Instead, “I'm going to do my best to bring the best of myself to a cause that's vitally important.”

But, like always, she'd welcome the right script.

“The parts that I'm offered are often old, decrepit women,” she says with obvious distaste, “and I refuse to play those roles! There are actors who WILL, and do it very, very well. I could do it rather well, too. But I'm not going to. I want women my age to be represented the way they are, which is vital, productive members of society.”

Until then, “I still think of myself as a working actress, and a very ordinary person,” Lansbury declares. “But the way people perceive me — well, it's lovely to have people remember and appreciate what you do. It's pretty wonderful.”

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