"You have no idea," she grins, rolling her eyes at an endless series of faux pas. "Sacha is such a funny guy but he's going to finish me in Hollywood, if he hasn't already.
"He's the man who goes up to Cate Blanchett at the Golden Globes and asks what she does for a living. I mean, really? ... "
The diminutive Aussie firebrand, who lends her vocals to this weekend's kid-friendly release, Rise of the Guardians, was left especially red-faced when attending a dinner thrown by Warren Beatty, with her other half.
"We're having a lovely time, eating dinner with Warren, his wife Annette Bening and some other lovely people and Sacha turns to Annette and says, 'You should have won an Oscar for The Kids Are All Right'. And she stares at him and replies, 'You don't know who I am, do you?' Turned out, it was Diane Keaton.
"So he doesn't know what to say, so wisely, he asks, 'What was it like to make love to Woody Allen?'"
Along with fellow Aussie expat superstars Naomi Watts, Guy Pearce and Chris Hemsworth, Fisher (36) cut her teeth on Home and Away, pestering Alf Stewart while loitering round the Summer Bay Surf Club.
But when the provinciality of the soap opera proved too restrictive, the Perth-raised beauty got a one-way ticket to Hollywood and landed a starring role as Vince Vaughn's homicidal love interest in comedy smash, Wedding Crashers.
Unfortunately, her career temporarily crashed soon afterwards.
"I couldn't get a job for a whole year," she explains. "It was pretty soul destroying. The reality is there's just not a lot of good material out there for funny women. So I was pretty depressed about it."
Isla sadly laments a missed opportunity to temporarily set up house in Dublin two years ago while shooting the medieval comedy Your Highness, with Natalie Portman and James Franco.
"I was so disappointed about that but there were a couple of creative differences in the script so Zooey Deschanel took the part and did an amazing job.
"We were going to be splitting our time between Belfast and Dublin so myself and the family were all really looking forward to a couple of months in Ireland. We had the houses sorted and everything, deposits down. It wasn't meant to be, but it'll happen again down the line. Mark my words, I'll be back."
A couple of minor follow-up roles preceded her first lead job in Confessions of a Shopaholic, a chaotic, somewhat flawed interpretation of Sophie Kinsella's bestseller that nevertheless, cemented Isla's status in the A-list. But after the birth of daughter Olive in 2007, the new mum decided Hollywood could wait.
"I just think I'm better off doing supporting work that doesn't require much time away from my kids," Isla, who since had her second daughter with Cohen, Elula, two years ago, explains. "They're only going to be young once, I want to be there for that."
Rise of the Guardians, where she plays a feverishly endearing Tooth Fairy, is her third foray into animated territory, after Horton Hears a Who and last year's reptilian western, Rango.
Intentional career moves for her girls? "Actually, they've never seen any of my films. I'm not even sure they know what I do. I'm at home so much, they think I'm a stay-at-home mom."
Also featuring Alec Baldwin, Chris Pine and Hugh Jackman, Guardians sees figures of imagination, Santa, Easter Bunny and the Tooth Fairy join forces when the boogey-man, Pitch Black, threatens ever-lasting fear on the children of the world.
"It's a beautiful, exciting story, like The Avengers for kids. I love the twist on all the characters, in appearance and demeanour. I love that Tooth is like a hummingbird and Easter Bunny is this surly Aussie. And it's the kind of film you need to see a few times, there's so much going on screen. It's phenomenal.
"And I love doing animation. I can rock up to the studio in sweat pants, greasy hair, and put on my squeakier than usual voice. It's the best job ever."
While she claims her kids never catch her movies, surely Rise of the Guardians will simply prove too enticing, particularly for a five year-old smitten with Santa and the Tooth Fairy?
"Well no one's lost a tooth as of yet, although I'm sure it's coming and we celebrate Hanukkah, as well as Christmas in our house so there's so many presents, from family and grandparents. I think the notion of Santa is still a little foggy for them."
"But I am a little concerned when it gets to that stage though, I feel weird about lying to my kids, telling them Santa and the Tooth Fairy are real. I haven't done it yet and honestly, I don't know how I feel about that."
She will next be seen in in Baz Luhrmann's hotly anticipated, updated version of The Great Gatsby with Leonardo DiCaprio and Carey Mulligan.
Try as she might to shrug their 'power couple' label, calling it 'beyond ridiculous', the industry has been waiting patiently for a collaboration between Fisher and Cohen. Is it ever likely to happen?
"I'm obviously such a huge fan of what Sacha does, but we work together in so my other areas in our life that I'm happy to do my own thing and have him do his own thing.
"Again, when the kids are older, it could happen.
"Right now, I've already lived with the handle bar moustache, the Bieber flick, the dictator finger. Would I want to be involved in one of his projects? That remains to be seen."
Rise of the Guardians is in cinemas from tomorrow