Nicole Kidman has squirmed as interviewer James Corden asked her about one of the most memorable moments of her recent Cannes Film Festival jaunt.

Kidman had four films showing at the iconic film festival, but it was a surprisingly intimate moment with country singer hubby Keith Urban on the red carpet that had celeb watchers talking.

In photos seen around the world, the pair appeared to share a very intimate moment, embracing as Urban whispered something in his wife's ear that seemed to make Kidman overcome with emotion, reports News.com.au.

The question on everyone's lips: What on earth did he whisper to her?

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Host James Corden asked Kidman exactly that on the latest episode of The Late Late Show.

"Oh, just something ..." she said with a giggle.

"I love Keith. It's a weird thing actually, because when you're in that massive - all those photographers and the noise and everything - when you create intimacy, like a little bubble, it's really emotional. Just having him there was emotional," she continued.

That wasn't quite enough for Corden, who pressed her further.

"I'm not telling you what he said!"

Kidman appeared on the show alongside Game of Thrones actor Kit Harington, who opened up about moving in with girlfriend Rose Leslie.

A practicising Catholic, Kidman revealed that she's perhaps more traditional than some might expect, pressuring the young actor to propose to his partner.

"Are you gonna get married? I just think it's kinda nice, if you're gonna live together, at least get engaged. I'm just all about, hey, what about getting engaged and THEN living together?" she said.