Image copyright Universal Image caption Sandra Bullock voices Scarlet Overkill - the world's first female super-villain

Sandra Bullock is one of a handful of Hollywood stars joining Despicable Me's lovable, pill-shaped creatures the minions - as they take centre stage with their own movie spin-off.

It's been 17 years since Sandra Bullock last starred in an animated film, lending her voice to Moses' sister Miriam in Dreamwork's Prince of Egypt.

But when asked about her decision to join the cast of Minions, she couldn't have responded quicker.

"My son," she smiles. "I wanted to do something that he could see, that I was in."

Bullock has an adopted son called Louis, who travelled with her to London ahead of the movie's world premiere.

It sounds like a glamorous life for the much-papped five-year-old, but Bullock points out that "he doesn't know what I do.

"I've started telling him that I go and make stories, but he doesn't quite care what that's about - nor do I want him to."

Image copyright Getty Images Image caption Bullock says we love the minions because they "have such sweetness and good will"

So he doesn't realise his mum is one of Hollywood's highest-paid movie stars?

"[The phrase] 'famous movie star' doesn't really mean anything, does it? We're supposed to embarrass them! You're not supposed to have respect from your kid."

She gushes that her son loves physical humour and animation, and because she loves to hear him laugh, joining Minions was a no-brainer.

The film is a spin-off prequel to Despicable Me and Despicable Me 2. The latter alone took $970.8 million (£616m) at the worldwide box office, and it was the top grossing film of 2013 in the UK.

In Minions, it transpires the yellow creatures have actually been serving despicable masters since time began, from dinosaurs to vampires - but because of their accident-prone ways, they have been completely unsuccessful at keeping their jobs.

Without a master to serve, they are driven into a depression, before one of the more proactive minions, Kevin, hatches a plan to find someone new to work for.

Along with fellow minions Stuart and Bob, they head to a villain convention which leads them to their next potential master - Bullock's Scarlet Overkill, the world's first-ever female super-villain.

Overkill proves a contrast to the Oscar-winner's usual roles, which have given her the reputation of America's sweetheart.

"I still don't know what that means, but I'll take it," she laughs.

Image copyright Universal Image caption The minions are all voiced by director Pierre Coffin

"As an actor, being a villain is a great thing. Everyone wants to be able to do that. And we're human beings - we're crabby - so to be able to be mean the entire time you're working? It's a great release."

Although she says getting into character wasn't hard - "Oh crabby's easy for me. That's a no-brainer" - the actualisation of Overkill was much more of a challenge.

"It was one of the harder things I've done. Every day was different and every day was a struggle.

"I'm so used to using a prop or a person or my body to sell something and I couldn't sell the character that way. I had to only use my voice, which was tricky."

"Just when I thought I did something well, I would hear, 'we need you to do it again, we didn't get it' - and I'm like, 'how could you not have gotten it?!'

"Then I realised, they're in the other room with their heads hanging low, not even looking at my body - they are just listening., Listening for the sound.

"It was not an easy road to get there, but I'm so glad I did it."

Image copyright AP Image caption Bullock won the best actress Oscar in 2010 for her role in The Blind Side

Pierre Coffin not only directs the film but also voices the entire minion population, including Kevin, Stuart and Bob.

"I'm not an actor in any way but I try to find what could differentiate and identify the three minions. In the performance it's me trying to find the proper melody for whatever the emotional state of the character is in that moment, with whatever words sound funny in the tone."

Coffin's 'Minion-ese' - a mixture of languages including French, English, Italian and Spanish - is a huge part of the movie's charm.

He and co-director Kyle Balda said they had a lot of fun directing their roll call of guest stars including Jennifer Saunders, Steve Coogan, Michael Keaton and Mad Men star John Hamm - who Bullock describes as "delicious".

The appeal of the minions is apparently unbounded, with Universal already confirming Despicable Me 3 will be released in June 2017.

"It's their innocence," says Bullock on their level of popularity. "They don't mean to harm or make something collapse or explode. It's accidental.

"They have such sweetness and good will. And we all understand them. No matter where we come from, we all understand the minions."

Minions is out in UK cinemas on 26 June