United Kingdom, March 9 -- Cate Blanchett thinks "profound trust" is the key to a successful marriage.

The 55-year-old actress has been married to playwright Andrew Upton, 59, since 1997 and insisted that longevity should come with the "expectation" that each partner will never "truly know" the other.

She told The Observer: "Yes, it's all about the things that are not said, which is really interesting to play. I think it's a fascinating way to look at a marriage now because it's meant to be all about honesty, having everything out there.

"But what does that mean for desire and what does it actually mean for trust, if there are no secrets that you're prepared to keep?

"Every marriage is different, but the ones that last are based on a ...