I first met Maggie on the Harry Potter films, when she played Minerva McGonagall in Goblet of Fire. It wasn’t a very big part but she was always very notable, always poised in whatever she did – even when wearing a comedy hat.

She brought a lot to the part, and to the set – and one of those things was terror. Maggie never bit me, but I think if she did bite you, you stayed bitten.

There was one day of filming when the set was in a thing we called the flight shed, which was an enormous barn of a place made entirely of corrugated iron. In winter it was absolutely freezing. There we all were, all toeing the line and waiting for Maggie – who was not late, she was never late – and she walked in through the doors and said: “Well, it’s freezing in here, and I’m not going to shoot until you’ve warmed it up”, and off she stomped. This was not in any way fuzzy or warm, this was a command, and you’d better take it seriously. Almost immediately on her heels came one of the producers, his face the colour of parchment – somehow he managed it. Maggie was just very clear; she knew what could be done and what should be done.

She had a look that could express disappointment or disbelief when she was being directed. A sense of “he’s asking for this, but he’s going to get something bigger and better, and I hope he realises he’s a very lucky boy”. And I did.

One day I saw that look, and I don’t know what possessed me, I ran down the length of the set and yelled: “You think I don’t see, but I do!” It wasn’t to tell her to stop – Jesus Christ, no – I wanted to tell her that I did perceive what she was doing with the role. By that time, in a career of that length, people know what they are doing and she’d done it for a very long time. She was an institution, and you don’t question an institution. It’s like questioning Callas.

So she was very, very formidable, but she was also very sweet and very kind with the children. She was such a wonderful, natural comedian that if she found herself with a child who had comic possibilities, she would kind of drape herself around this child, and there would be wonderful comic little insights that would come out of it.

She will for ever be Miss Jean Brodie and in that role perfectly revealed what was impressive about teachers for young people, but what was also secret about them. But she was a far bigger and more important figure in British culture than that. In Gosford Park she showed how much could be revealed about a character in the smallest of details, like noticing when her breakfast tray was brought into this magnificent bedroom that the marmalade had been shop-bought. The wonderful barbed aside, she was brilliant at those.

I would absolutely say, without a doubt, that she was one of our greatest actors. She was an extraordinary theatrical talent, with an intrinsic musicality and grace to the way she moved and spoke. To get all those qualities in such quantities means we have lost an inimitable voice. There could only ever be one of her.

As told to Alexandra Topping

• This article was amended on 3 October 2024. It was in Gosford Park, rather than Downton Abbey, that Maggie Smith delivered the remark about shop-bought marmalade.