How did they film baby Noah in child abduction drama The Cry?

Turns out The Cry had a few teething problems (excuse the pun). In fact, they actually had to fire a couple of babies for growing too quickly. But – in Jenna Coleman's words – they struck on a pair of six-week-old twin "genius actor babies" who gave the performance of their (very young) lives.

Coleman was joined at a screening in London by director Glendyn Ivin and executive producer Claire Mundell, where they shared some behind-the-scenes secrets about filming with the baby boys...

How were the babies chosen for The Cry?

"It's a pretty tricky thing to find a fresh baby," Ivin said.

And then even when you think you've cast the perfect baby, things can go wrong. The director explained: "We actually had cast some other babies and another set of twins quite early on, and you think you've ticked that box: 'Right, we've got the babies.'

"And then they came in for some wardrobe or something, and of course they'd grown. They were little – and now they were big.

"I remember walking out – 'These babies are too big. I just can't do it.' So then someone has the job of: you've got to go out and find some new babies."

But whoever went out to re-cast the role of Noah got lucky, because they found twin boys Noah and Oliver Rennie. The babies were born to a mother-of-six who agreed to take part in the production. Though they're not identical, the boys from Kilmarnock in Scotland are the same size – and look extremely similar.

"We went and visited her in her house and there was a bit of – there was time pressure, let's say that," Ivin said. "Because also I didn't want them to grow any more. I realised how desperate we were.

"I do remember walking in, seeing the babies, and they were in the bassinets there and the mum was obviously pretty cool, and going through the niceties of introducing yourselves and: can we have your children, essentially. And: do you have a passport? Can you sign this?

"But at the same time trying to explain the process of what we are about to go through and what's the story about. There is a process that you've got to go through so the mother feels very comfortable and the family feels comfortable."

While Oliver and Noah were the stars of the show, they weren't the only babies used in the production. "Through the show there's probably seven or eight different versions of Noah and I don't think anyone's twigged it yet about which baby's which," Ivin says. "Because obviously we're in different places and countries."

• For an in-depth look at the casting process for TV babies, read our feature: Where do babies on television come from?

How do you deal with babies on a film set?

"It is tricky, working with actors – including babies," Ivin joked. "The twins Noah and Oliver, they were six weeks old and it's pretty real. You put them down, they cry, everyone gets very alert, and you film, and then you take them off and they're not crying any more."

When working with babies on set, more often the challenge is to stop them from crying for long enough to film.

But on this project it was a case of waiting for Noah and Oliver to start bawling – as (of course) directors aren't allowed to upset babies on purpose.

"We shot so fast, it was like – we're crying? Go. just do it. Go," Ivin said. "It's pretty interesting how quickly a crew falls into line when there's a crying baby on set. We all know if you've ever had kids or been around them, two minutes of crying can feel like half an hour very quickly so everyone wants that baby dealt with and off set and moving on."

Mundell added: "I recommend all producers get babies on set, even if there isn't one in the story, because it really makes everybody move quickly."

"Generally when you go into shoot a day, you're pretty prepared, you've got a plan," Ivin said. "But whenever it was like: Noah is on the ground crying – you're kind of at the mercy of a six-week-old baby... you've got to cross your fingers and pray to the baby gods."

Because filming took place in Melbourne first, the cast didn't actually meet Noah and Oliver until they arrived in the UK for the final stretch of the shoot. The twins were then brought in for five weeks of filming, which took them to Glasgow and to an aircraft interior set in Maidenhead for three days.

"We really felt like we had got to Melbourne by the time we had finished those three days didn't we?" Mundell said. "And fortunately baby Noah is a twin so we were always able to interchange the babies. Oliver's got hair but we just put a hat on."

On working with the babies' mother, she explained: "Well first she had six babies, so she's actually quite relaxed about having her child on set.

"Obviously all the protocols that you'd expect were followed very strictly, and actually the baby on set was – I think we had actually a rule of no more than 15 seconds between baby on set and action, and that's pretty much how it occurred. We worked with a stand-in doll in between when we were setting up the shot."

How did Jenna Coleman prepare to play a new mum?

The actress may not have a baby of her own, but she did plenty of research for the role of new mother Joanna. As part of her preparation she spoke to a midwife, called on the advice of friends with kids – and also took a very awkward trip around Melbourne with a pram.

"Glendyn sent me on a trip with a stroller," she explained. "I went out in Melbourne with a pram, because it was kind of getting used to how people treat you with a baby.

"And it was really good except for a couple of people came up for some photos and then looked in the pram, and then there was just an empty bottle of water in the pram, to which I then had to explain the director made me, so then I got in a hole."

She added: "I went into a store and they prescribed me post-natal face cream, to which I didn't have the heart to explain – and I just bought it and left."

But has it put her off having a baby herself?

"No it hasn't, but it's opened my eyes up to the realities of motherhood, and opened my eyes up to a lot of what my friends are experiencing perhaps at the moment," she said. "And really just that people don't really talk about it that much, I think. But yeah – it's not put me off forever or scarred me forever!"