The House With A Clock In Its Walls sees Jack Black and Cate Blanchett play a warlock and witch in a race against time to save the world. Georgia Humphreys chats to the Hollywood duo

CATE BLANCHETT and Jack Black are trying to talk to me without moving their lips. Except the word 'lips' is actually really tricky to say in this manner, and they soon crack up laughing.

Some might think these two Hollywood stars - one (Black) known for more comic roles, the other more serious - are an odd pairing. But it's clear from their joking around that they had fun filming together. And they're hilarious to watch as witch Florence and warlock Jonathan, in new fantasy adventure A House With A Clock In Its Walls. Particularly memorable are their one-liner insults to each other, some of which California-born Black, 49, says were improvised.

"'Mush brush' was my favourite," Australian Blanchett, also 49, recalls with a grin. "It's so easy to think up insults for men with facial hair."

From Steven Spielberg's production company Amblin Entertainment, the film is based on John Bellairs' 1973 novel, and starts with orphan Lewis (Owen Vaccaro) moving to live with his uncle in a creaky old house.

"What I love about Florence, and Jonathan as well, is that their magic is broken, because they've both had tragedies in their lives," says Blanchett, an Oscar-winner whose memorable roles include The Aviator, Carol, and the Lord Of The Rings trilogy. "The strength, ingenuity and weirdness of this little boy that comes into their lives gives them courage to use their magic again."

Lewis quickly realises his new home is far from ordinary, as Florence and Jonathan introduce him to a world full of spells, and a quest to discover the source of a mysterious tick tock noise coming from within the walls. When the trio realise that, if the clock goes off inside the house, it's going to send time backwards to before human beings even existed, it could well be down to Lewis to save them all.

This, the stars say, feels very timely. "My youngest, Tommy, is very passionate about environmental issues and he insists I'm not allowed to buy a gas guzzling car any more, so we've switched over to electric," reveals Black, a father-of-two known for films such as School Of Rock, Jumanji and The Holiday. "The next generation is already taking the weight of the world's problems on. You can see the gears moving in their head about how to save the planet and it's something I think we should encourage."

Blanchett, who has three biological sons and one adopted daughter with her husband, screenwriter Andrew Upton, agrees there's anxiety about the way the world is going at the moment. "The notion of a doomsday clock, and time is running out... We are running out of time, in a lot of ways. When we talk about current events around the table, our 10-year-old wants us to stop talking about it, because he has to live in the world."

A notable line in the film suggests how excruciating parenting is - that if you're not in desperate fear the whole time, you're probably doing it wrong as a parent.

"That's one of my favourite scenes in the whole movie," notes Blanchett, "because you get this strange, broken, pseudo father-son relationship. You have to confront your own childhood when you become a parent and that's what you see Jonathan doing. By the end they have this fantastic relationship, but it's hard yards getting there."

"There's a fear, being a parent, that you're going to mess it up, that you're not going to do it right, that I definitely was able to tap into... a lot of times where I feel like the insane uncle that suddenly has to really be a father," elaborates Black, whose wife, Tanya Haden, is an American artist, cellist, and singer. "There are mysteries to that responsibility that no-one can really prepare you for, and that's a big part of what the movie's about for my character."

The weird and wonderful adventure that is The House With A Clock In Its Walls is certainly a little jumpy at times.

As director Eli Roth, known for features such as Hostel and Cabin Fever, puts it: "If you're really into scary movies and you want your kids to be into scary movies, then this is the movie that you bring them to, as a kind of gateway movie."

Blanchett, who reveals she is a big horror fan herself, was excited to be doing a film her kids can watch. "My eldest son is particularly interested in film," she says. "When I got the script, he went, 'Eli Roth! Oh my god, you've got to do it'."

As for Black, his kids don't really like watching him in anything. "They much prefer to see Will Ferrell, or anyone else," he quips, flashing one of his wide-eyed smirks. "I think that's the natural way of things. Kids are embarrassed of their parents, and when we're out in public they like to walk 100 yards ahead of me and pretend like we're not together. I know that they have enjoyed secretly some of the things I have done. But they would never say."

During the press tour for Ocean's 8 earlier this year, Blanchett made headlines for saying that female-led films are often misunderstood by male critics. With this latest project, is what kids think more important than what the critics think?

"I think critics should know how to write, and have a sense of context and also place their own personal taste outside of what they're criticising, whether they're female, male, transgender, children, whatever," Blanchett suggests earnestly. "The more diverse voices there are, the more you're going to get a sense of many, many different perspectives. When they're all white men, you're probably going to get a similar kind of view about what works and what doesn't.

"There are so many different films of different scale and ambition made, and it would be great to think they could all be seen for what they are."

* The House With A Clock In Its Walls is in cinemas now.