It's safe to say that the success of James Cameron's Titanic hinged on Kate Winslet's chemistry with Leonardo DiCaprio.

Thankfully, the two young stars instantly connected, making the 1997 epic the iconic classic it is today. And over 20 years after the film's release in theaters, their bond is still a marvel, which Winslet gives new insight into for a new featurette, Titanic: Stories from the Heart, found on the bonus disc of the new 4k Ultra HD release.

"Once I started working with Leo, we were able to kind of find our own rhythm. And it's amazing to kind of look back and think about it all over again," Winslet remembers in ET's exclusive peek at the upcoming release above.

She recalls that they "clicked immediately, right away." "He was this kind of mess of long, skinny, uncoordinated limbs. And he was just very free with himself, and he had this effervescent energy that was really magnetic," she adds. "And I remember thinking, 'Oh, this is gonna be fun. We're definitely gonna get along.' And we just really did. We just really did."

Gushing about her former co-star and close friend, Winslet praises DiCaprio for his way of seeing their characters, describing the other star as a "ferociously intelligent man."

"He was then very, very smart, very, very curious. So he was really fascinated with the period, the details to do with the boat, the lower classes, where those people had come from, how those people had paid for their tickets," she shares. "We connected on so many levels. That sense of focus on the craft and still really caring deeply about that to this day."

The close bond they formed on the Titanic set is still alive today. Winslet shares how often the two connect in real time, saying, "And we'll always just make that call right away. There's no like, 'Hang on, I'll call you tomorrow.' It's instant. And that's actually really something."

"You know, if you think about it, in the world that we live in now, to have friendships that bind you, and that shared history, it's really something," she concludes.

To think there had been a chance that DiCaprio wouldn't have landed the role of Jack opposite Winslet as Rose! While chatting with ET last year, Cameron revealed that he had to "sell" the young actor to the studios at the time. 

"Look, we were leaning toward Leo, but I hadn't seen him do anything," Cameron told ET. "And he didn't want to read [for the part]. He said, 'Oh, I don't read.' You know, I'm like, 'Well, are you a little diva? 18 years old? 19 years old?' 'No, I don't read.' Like, 'OK, really? See ya. Wouldn't want to be ya.' So, I called his bluff, right? And he went and read. And he was spectacular, of course."

While DiCaprio may have won Cameron over, the studio wasn't convinced. "But now, I had to sell him to the studio," the director recalled. "And they were like, 'Based on what? [What's Eating] Gilbert Grape? Come on, we're gonna hang this giant movie, romantic film on Gilbert Grape? Give me a break.' So, you know, then I had to talk him into it. But see, Leo wouldn't let me film [him reading]. So, I was like, 'OK, you just have to take my word for it.'"

Needless to say, it was a "tenuous" journey for Cameron. "It shows you how fragile the whole thing is, this whole moviemaking thing," he said, before later adding, "I'm pretty careful about my casting." 

"I mean, I certainly can't imagine it being anyone other than Leo. But also, I was not party to any of those conversations," Winslet said of Cameron's revelation about DiCaprio almost not getting the part. "I mean, like, how could it have ever not been Leo?" 

That said, "I am enormously proud of it," she said of the film, adding, "I have always maintained that Jim is a complete genius. He's an extraordinary filmmaker and he knows exactly what he's doing. His storytelling, his scriptwriting -- he's extraordinary at creating not just strong roles for women, but women who are at the center, who are leaders, you know, who lead with integrity." 

Fans can dig into more behind-the-scenes goodies when Titanic drops in 4K Ultra HD for the first time on Dec. 5. It'll be available in a new two-disc set that also includes a Blu-ray disc with five hours of new and legacy bonus content, including the new Stories from the Heart featurette.

A limited-edition collector's boxed set is also available, featuring a coffee table book, a blueprint of the Titanic, movie prop reproductions including ship menus and Jack's fateful boarding pass, and sheet music for "My Heart Will Go On."

The bonus disc also features new behind-the-scenes content with producer Jon Landau, taking audiences behind the making of Titanic and its trailer.

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