Dame Sheila Hancock feared she had dementia after struggling to remember her lines.
The 87-year-old actress admitted her mind went blank while she was on set filming her Sky One drama 'A Discovery of Witches', but she was relieved when she returned the next day and was able to recall the script.
She told Radio Times magazine: "It was terrifying, not being able to remember anything and your brain panicking and trying to find the words."
After the moment of memory loss, Sheila has been thinking more about the prospect of being diagnosed with dementia.
Meanwhile, the star - who was diagnosed with rheumatoid arthritis three years ago - previously appeared on 'Great Canal Journeys' and told Gyles Brandreth, 72, that she feared she might only have months to live.
However, she has now explained how her comments weren't related to the condition, which can potentially cause life threatening problems.
She explained: "This wasn't to do with my illness - although you can die of it and I do have to think about that - but just at my age, you're unlikely to live many more years and that weighs heavily if, like me, you're greedy to learn new things."
And she also addressed her thoughts on the afterlife, and insisted instead of life after death, she believes people - including her late husband John Thaw, who passed away in 2002 - leave "vibes" behind when they die, and she hopes to "leave a few good vibes".
She added: "When I think about the friends who've died, and sadly there's a lot of mine, there are certain people you can virtually see.
"I can see John. I can see him standing there. I do think that people leave 'vibes' behind, and the best that I can hope is that I leave a few good vibes, as well as the bad ones."
