Helena Bonham Carter took her first starring role on screen in Lady Jane in 1986. That was followed by roles in films including A Room With A View, Fight Club, Sweeney Todd and Ophelia opposite Mel Gibson’s Hamlet. She has appeared in several movies directed by her partner Tim Burton, including Planet Of The Apes and the Red Queen in Alice In Wonderland. In The King’s Speech, she plays King George VI’s wife Elizabeth – the Queen Mother – who seeks out an unorthodox Australian speech therapist to help her husband’s stammer. Following a screening at the 54th BFI London Film Festival, the actress spoke about meeting the Queen Mother her in real life.

Did you know much about the king’s stammer beforehand?

I VAGUELY knew he had stammer, but was unaware how chronic it was. What this film shows is a whole completely new angle on a very famous period of history for us, the abdication, and the pressure on this man. It’s a story about the most reluctant king. I suspect Edward VIII didn’t want to be king either. It’s the duty, the responsibility, the sheer hugeness off the job. I certainly would never want to be royal, even though I effortlessly am at times. In fact that was partly why I did play it, because I knew that I could indulge in being queen.

How was it being royal?

We could behave outrageously and it would be accepted.

I’ve played a few queens lately. It’s enjoyable just to put it on and pretend but then you can take the crown off and sling it across the room. You don’t have to be the nice smiling queen. The Queen Mother was extraordinary because she was a professional public figure, an expert at it. She had the character and the confidence, but she married a man that was not born to be king and really wasn’t constitutionally meant to be king, just in the way he was built. Luckily he drew upon her confidence where he lacked it. It was a really true partnership.

She was the classic woman behind the throne.

Bertie is terrified of public speaking – are you the same, do you suffer from stage fright?

I don’t like making speeches. It’s not my idea of complete joy, sitting up here, as much as you are all lovely... I’m a very introvert actor who likes putting on other people’s clothes and pretending to be somebody, which a completely crazy choice of profession. I’m fairly introverted, but at times have spasms of extraversion. So no, I’m not very good at public speaking. I have absolute every sympathy with him.

Do you do research when playing a real character or just go by the script? And do you try these people on at home to see how they are going to play?

Yes, otherwise it wouldn’t be any fun. Certainly in this case when you’re playing a real person, you have a real responsibility. I read a bit but I didn’t actually have that long, because I was playing a witch in Harry Potter at the same time.

How was making The King’s Speech and the Harry Potter film side-by-side?

Actually my son, who was six at the time, said “Mummy, do you have to play the witch or the Queen tomorrow?”. I said that’s a good question, as long as I don’t get them mixed up. The witch, Bellatrix, is quite exhausting because she is all screamy and a bit of a hooligan so it was quite nice to have the weekends off, being a bit more restrained.

Anyway, you do all the reading, but then ultimately you have to serve the story. Obviously I don’t look like her, I hope. Not that I mean that in a disrespectful way, I mean her later years.

What do you mean?

Oh dear, dear, dear! I might as well just die now.

You just try and capture some kind of essence of her.

Did you ever meet the Queen Mother and did that inform your role?

I met her because she came the premiere of Room With A View when I was very young, all those centuries ago. She was great at being gracious and had that cloud of charming vagueness. Underneath it, having read about it, she had a huge amount of inner strength. Cecil Beaton said that she was a marshmallow but made by a welding machine. I thought that I should try and get that duality.

● The King’s Speech opens in cinemas on Friday