Dame Maggie Smith has admitted that her reputation as being terrifying to work with may well be deserved, though her theatre friends believe she is "a genius".
The actress appears as Mary Shepherd in The Lady In The Van, a story by writer Alan Bennett about an elderly woman who parked her van - and home - outside his north London house in the late 1960s before moving it on to his front garden, where she stayed for 15 years.
The tale has been told in book form and on the stage - which Dame Maggie starred in - and now director Sir Nicholas Hytner has brought it to the big screen.
Dame Maggie was the only choice for the role of Miss Shepherd, Sir Nicholas and Bennett decided.
Sir Nicholas said: "There is nothing she can't do, and there are 20 options for every single moment. Her imaginative energy is extraordinary - as well as her physical energy, I have to say. She's constantly driving herself to do better."
Her friend and long-time collaborator, producer Robert Fox, described her as a "genius", saying: "There is nobody as accomplished as she is. I think her talent is innate, but it's honed over years and years in theatre - 60 years. You can't just do it, you have to learn how to do it."
Along with her position as arguably Britain's finest actress, she also has a reputation for being difficult, or even terrifying, to work with.
Dame Maggie is aware of her notoriety, but puts her fear factor down to one thing - being scared herself.
She told the Telegraph magazine: "Obviously at some times I have been like that - the awful thing is, I'm sort of very aware when I'm being difficult but I'm usually ... so scared.
"And that's shaming, at the age one is. Because every time I start anything, I think, 'This time I'm going to be like Jude (Dench), and it will all be lovely, it will be merry and bright, the Quaker will come out in me'.
"But it never works. Jude has a wonderful calm, it's very enviable. I think it would be hairy if she let fly, but I've never seen that.
"But it's gone too far now, to take back. If I suddenly came on like Pollyanna, it wouldn't work - it would frighten people more if I were nice. They'd be paralysed with fear. And wonder what I was up to."
Now 80, Dame Maggie admitted to still getting nervous when performing. She said: "When you're young you're just so thrilled to be doing it; of course you're terrified, but as you get older it's because you realise how difficult it is to do. When you're young it's excitement and terror. Now it's just terror."
She also said it was the right time for Downton Abbey, now in its sixth and final series, to come to a close.
She said: "It was one of those odd things - nobody knew it was going to go careering on as long as it did, and it was jolly exhausting.
"But I did much less in the last few months, so I've sort of faded rather nicely. I just had dumb arguments with Penelope (Wilton), which was quite fun. We had a lovely time. She's such a great actress - we used to shriek with laughter."
:: The full interview is in today's Telegraph magazine. The Lady In The Van is out on November 13.