Kristen Bell gets nasty when the CHiPS are down

When US actor Dax Shepard was casting the role of his character's ex-wife in the new movie CHiPS which he wrote, directed and starred in, he initially didn't want to give it to his real-life wife Kristen Bell.

The comic couple have popped up in each other's movies before, like When In Rome, Hit And Run and Veronica Mars.

"He sat me down and said, 'I'm not positive you can be as unlikeable as I need you to be for this', which I took both as a compliment and an insult," the 36-year-old US actress said with a laugh at our interview at the London West Hollywood hotel, adding "because I can be very unlikeable".

And she is. In CHiPS.

But in real life, Bell - who famously voiced Anna in the Disney animated hit Frozen - couldn't be sweeter.

In the buddy-cop action comedy which opens here tomorrow, she plays the evil ex, Karen, who divorced Shepard's California Highway Patrol motorcycle police officer Jon Baker.

He's so focused on not giving up on her that he stays in the tiny guest room of the mansion they once shared, where she still lives, and still goes to couples' therapy - by himself.

Meanwhile, she has a new boyfriend and parades around in a Baywatch-type swimsuit with cleavage down to there.

Bell is in just a handful of scenes, but you are likely to remember her.

The CHiPS story is based on the eponymous 1970s TV series and revolves around rookie Jon and his undercover FBI agent/partner Frank "Ponch" Poncherello (Michael Pena) as they investigate a multi-million-dollar heist suspected to be an inside job. Their unlikely bromance, motorbike chases and snappy dialogue make for an entertaining ride.

At our interview, Bell showed that Shepard is not the only funny one in the family.

"Someone said, 'Your husband gave you a lot of cleavage', and I was like, 'Please hold'," she said.

"My husband didn't make me do anything, because he can't.

“I didn’t pump for about a day, and if you don’t pump, your boobs go out and then they go up.”Kristen Bell

"I needed to present Karen as superficial, who considered herself a trophy wife. She wanted to be with Jon because he was winning trophies. But when he wasn't winning any more, she was like, 'I am out, because I am the prize'."

It helped that Bell was breastfeeding. The couple have two daughters, aged four and three.

Said Bell: "I didn't pump for about a day, and if you don't pump, your boobs go out and then they go up. It was pretty painful, and my husband was like, 'Are you okay?' And I was like, 'I need to give Karen an aggressive boob job'.

"And I said to the kids, 'If you are hungry, find something else. Daddy is making a movie, and this is everyone's priority now'."

Of her 10-year relationship - they married in 2013 - with Shepard, she said: "In high school, I was like, 'I am going to marry a Greenpeace worker'. Then I get this motorcycle enthusiast who is made of testosterone, and does nothing but tell pervy jokes, but it works."

The first time they met - at a dinner party - he didn't make a good impression because she said he talked too much.

Two weeks later, they met at a hockey game and bonded over chewing gum, of all things.

Shepard texted her a week after and the rest is history.

Asked what makes her so open about her personal life, Bell - who revealed last year that she has received help for depression and anxiety replied: "How is your life interesting or my life interesting if I give staged answers that no one cares about?

"It's not fun for you, it's not fun for me because I am rehearsed, it's not fun for the reader because they are reading nonsense.

"I believe in sharing my entire story, the good with the bad.

"I loathe the curated Instagram life where you are just pretending everything is perfect."