The 33-year-old actress - who starred alongside Leonardo in 1997 disaster movie 'Titanic' and reunites with him in new film 'Revolutionary Road' - claims the pair are so close they are like an old married couple.
In a joint interview, Kate, who was inspecting Leonardo's cut nose, said: "You've scratched the top of your nose! Oh my God, look at me fussing over your face. I've literally turned into a combination of your mother, your sister, and, I don't know what, your wife!"
The pair claim their close bond is partly rooted in them both being catapulted into superstardom after appearing in 'Titanic'.
Although Kate insists neither of them court publicity, Leonard understands why there is such interest in their private lives.
He told Entertainment Weekly magazine: "That's the interesting thing about being an actor. You are also a public figure. Early on, I made it my policy that a certain amount of publicity is good - you promote your job, you do your movie, you retain your private life, you don't divulge everything about yourself."
Leonardo, 34, has shied away from divulging the most intimate details of his life because he knows his fame will not last forever, so the only thing that really matters is the standard of his work.
He said: "All this noise and attention will absolutely deteriorate and there will be a new, fresh piece of meat for the media to focus on within less than a year's time."
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