Witch role suits Helena Bonham Carter best?

Bonham Carter (right) returns as Bellatrix Lestrange in Harry Potter and the Deathly Hallows Part 1

Some younger members of the Harry Potter cast have spent more than half their lives working on the boy wizard franchise.

But it has also taken up a large chunk of the lives of its adult actors.

"It's been quite a long journey, but enjoyably long," says Helena Bonham Carter as she demurely sips a coffee in a London hotel suite.

It is the day after the world premiere of Harry Potter and the Deathly Hallows Part 1, and Bonham Carter is taking obvious delight in the magical properties of caffeine.

"I've had a baby during this time," says the 44-year-old actress, otherwise known as malevolent witch Bellatrix Lestrange.

"I've been on and off it across four years."

Bonham Carter joined the cast in 2007's Harry Potter and the Order of the Phoenix, reprising her role in Harry Potter and the Half-Blood Prince.

"I love Bellatrix," says Bonham Carter. "She is very energetic.

"At times it was quite hard, because one of the films I did just after giving birth.

"I was required to jump up and down and do all sorts of things," she confides. "My pelvic floor, frankly, really wasn't up to it."

Bonham Carter will be seen again as Bellatrix in Deathly Hallows Part 2, due out in summer 2011.

"In the next one me and Julie Walters have a duel at the end and we both put our backs out," says the 44-year-old.

"My chiropractor says, 'You have to warm up before you do any spells.' There's a lot of throwing your arm round."

The erstwhile star of such Merchant Ivory costume dramas as Howards End and A Room with a View enthuses about her action scenes in Deathly Hallows Part 1.

Bonham Carter stars with Colin Firth (l) in The King's Speech, out in the UK in 2011

"When you're doing a spell in real life, you've got this bit of wood and you're shaking it around," she explains with child-like glee.

"Then when you see it on the screen you feel so empowered because they've got all the special effects and it becomes a snake.

"An action career - that's what I should have done!"

Bonham Carter is having a busy 2010 on the big screen.

First she appeared as the Red Queen in Alice in Wonderland, directed by her partner Tim Burton.

And she will be seen as a different kind of queen in historical drama The King's Speech, playing the future Queen Mother to Colin Firth's stammering George VI.

The King's Speech and the Deathly Hallows movies were filmed at the same time, which saw Bonham Carter juggling two roles.

"Potter was during the week and The King's Speech was my weekend job," she says.

"It was a bit schizophrenic. My little son would say, 'Mum, do you have to be the witch or the queen tomorrow?'"

Bonham Carter made her feature film debut in 1986 in Trevor Nunn's Lady Jane.

She was immediately offered the lead role in A Room with a View, going on to star in two more screen adaptations of EM Forster novels - Where Angels Fear to Tread and Howards End.

Her other early film work includes Hamlet, opposite Mel Gibson, and Mary Shelley's Frankenstein with Robert De Niro.

She went on to star in Woody Allen's Mighty Aphrodite and David Fincher's Fight Club.

Her films with Tim Burton include Planet of the Apes, Sweeney Todd and Charlie and the Chocolate Factory. She also had a cameo role in 2009's Terminator Salvation.

Is there a reason why she lands so many eclectic roles?

"I do lots of homework," the actress reveals. "I feel I've got the luck to have choice, but different things bring out different parts of one.

Tim Burton (l) and Helena Bonham Carter became a couple after Planet of the Apes

"If something's well written, or you're in the hands of a director who knows what they're doing, then a lot of the work is done for you."

"But you just do your homework, which is the fun bit for me."

This summer Bonham Carter filmed Toast, in which she plays the stepmother of renowned food writer and chef Nigel Slater.

The BBC One drama, based on Slater's best-selling memoir, sees her, somewhat confusingly, play a character named Joan Potter.

The young Slater is played by Freddie Highmore, who acted alongside Bonham Carter in Charlie and the Chocolate Factory.

"I play his stepmother, who he didn't particularly like but who cooked fantastically," Helena explains.

"Unfortunately she cooked so much, and so compulsively, that she fed his father too much and he died of a heart attack."

Harry Potter and the Deathly Hallows Part 1 is out in the UK on 19 November.

The King's Speech is released in the UK on 7 January.