But with only a couple of days to draw breath in Scotland, the normally poised Edinburgh-born actress is in a bit of a flap. What could be the cause? Has she forgotten to book a facial to polish her already flawless skin with crushed coral shells or some other exotic ingredient? Or a high-class manicure to cover up a microscopic chip in her immaculate nail polish?
"I haven't had a chance to do my laundry," she whispers conspiratorially with mock horror. "I haven't got any clean clothes."
Laundry? Surely a woman who grew up in a castle – well, spent some of her teenage years in one – and is now on the road to becoming Hollywood royalty has an entourage to carry out such tasks?
But judging by the way the 28-year-old falls about laughing at the mere suggestion, the small problem of clean smalls is all her own – and that's not likely to change if her budding acting career goes stratospheric.
And that seems increasingly likely these days. This week an episode of the hit detective series Cold Case was aired on US TV, which featured the former Fettes College pupil as a guest star. That has followed a starring role in a "mockumentary" about wiffleball, a variation of baseball, and a part in the US crime drama CSI: NY.
Sadly, one of her biggest breaks so far, a part in the Tom Cruise movie Lions for Lambs, ended up on the cutting room floor, leaving nothing to show for months of auditions and a week of filming.
But that, says Louise, is just showbusiness. "It is incredibly frustrating, yes, but something my agents have told me time and time again, once a job is done, let it go and focus on the next job. In this industry you have to keep your eyes ahead of you all the time."
Besides, the job gave her the chance to work with a Hollywood legend – Robert Redford, who was the director, who made a big impression. "He maintains control of his sets but he's very approachable – he's not very Hollywood at all. For example, I was calling him Mr Redford, sir, and he said, 'No, no, call me . . . Bob'." The way, she pronounces it, a la Blackadder, gives the impression she dodged such familiarity. "I couldn't call him Bob, I just kept calling him 'sir'".
Mostly, though, celebrities are such familiar sights in Hollywood that star-struck moments are few and far between. "Yes, you might see Al Pacino at the grocers but you would never go up to him and say, 'Oh my God, you're Al Pacino'. People look the other way when they see a celebrity."
Except for the paparazzi, presumably? Louise is careful with her answer. "You can seek out a lot of publicity if you want, depending on your choices. A lot of major celebrities don't have paparazzi following
'I'm hoping I'll be too busy making movies to take my law exams'
them." There is little chance of seeing Louise splashed across the red-tops, spilling out of a nightclub and a low-cut dress. She is dating, though she's coy about naming names, partly out of respect for her ex, LA lawyer Ronald Richards. Is the new chap an actor? "Maybe," she giggles.
Her days are filled with meetings at studios, auditions, reading scripts and photo shoots, while her evenings are spent cramming in her homework – she is studying for a degree in law.
A rare night out was in Glasgow last weekend for the Style Awards – her date was her big brother, David Hay – and she was thrilled to win. "It's my first award for anything, so it's really nice." And just being in a room with hundreds of people from the creative industries made her realise how much talent there is in Scotland, she says.
If her life sounds charmed, then that's because it is to a certain extent. She is as genuinely beautiful in the flesh as she is in her photographs, slender as a blade, with sparkling eyes and teeth and long glossy blonde hair. "I am a natural blonde but not this blonde," she admits, as she explains she'll be going brunette for Five Hours South.
Growing up, her family weren't exactly short of a bob or two – they own Melville Castle, where Louise spent some time as a teenager. However the main family home was Murrayfield, and the castle is currently a luxury hotel, run by its tenant, entrepreneur Steven McLeod. But even so, her tastes are down to earth – her favourite clothes shop in Edinburgh is the rather less than exclusive Miss Selfridge. "Americans girls always ask me where I get my clothes and I say, 'ha, you can't get this over here'."
But her life has been tinged with sorrow – her mother, Rachel Hay, died of breast cancer at the age of 53 when Louise was just 14. "She was an extraordinary, compassionate, selfless person," Louise says. Her death was a body blow to the closely-knit family – Louise's father, William, and sister Suzanne, as well as David – and in fact her siblings work for the family businesses, Linton Hay Property and the Hay Trust.
Louise does believe her mum's death spurred her on to pursue her own dreams – as an idealistic 18-year-old she turned down a 50,000 modelling contract to work with orphans in war-torn Zambia. She loved her volunteer work with a charity, nursing HIV children in a makeshift hospital but the dream turned into a nightmare when drunken rebels attacked the remote village and Louise and the other Westerners were forced to flee into the jungle. Louise eventually escaped with the help of local fisherman, who smuggled her on to her boat for a harrowing eight-hour sail across Lake Tanganyika. She has since written a book drawn from her experiences and that and a film based on the story are attracting interest from publishers and movie-makers.
In the meantime, though, there is the more immediate business of filming Five Hours South – an offbeat movie about a break- dancing Italian policeman. Louise, who plays one of two American sisters, doesn't have to do any break-dancing for the film, to her relief.
Then it's back to the States, hopefully for good news about a regular part in a major new TV series for which she has auditioned, the details of which are still under wraps. Then it's her law exams in early December. The final stage to qualifying as an attorney would be taking Californian bar exams, but it's a step she's hoping not to take. "I'm hoping I'll be too busy making movies for that," she laughs.