If anyone is familiar with these concepts from the Dramatica Theory, they’ll easily identify Rose as the MC (the “eyes” of the story… flashback or no flashback) and Jack as the Protagonist (“driver” of the story, making things happen).

If one of them was chosen by J. Cameron as both the “eyes” and the “driver”, then that character would be referred to as the Hero. Matt Damon’s characters in all the Bourne movies and his latest, “The Adjustment Bureau”, fit the bill of a Hero perfectly.

Rose is not the classical Hero because she doesn’t force the key events to happen. Jack does. He actively wins the trip tickets at a poker game, literally jumps onto the ship in the last second (no Jack on board, no love sub-genre, just a plain disaster movie), he talks Rose off the ledge, later invites her into his world on the lower deck where she discovers there are good emotions in life, not just upper-class boredom and once the iceberg hits, he leads the way through the water-filled ship.

There are scenes, in which Rose makes Decisions (standing up to her mother and reconsidering the marriage to Cal) or takes Actions (jumping back onto the ship as the rescue boat is being lowered) but that is a physical manifestation of her Character Arc. She would not have done that without Jack in her life. This is because in a Love Story (the Titanic sub-genre), the love interest (from Rose’s POV that would be Jack) is also the Impact Character to her Main Character. He is the one who presented her with an alternative POV and caused her to undergo a change. We, in the audience, identified with Rose from the beginning and wanted her to give up marrying into money. The iceberg was just the catalyst for her to make that decision faster. If the Titanic arrived in NY before she had made up her mind to leave Cal, may be she would have sacrificed her love for Jack, to ensure the means for her aging mother… so for the purposes of this blockbuster… good thing the big boat got blasted.

Last thoughts and examples:
1. I disagree – MC doesn’t always drive the story. Protagonists do. MC is just the character that the audience will inhabit for the purpose of each story.
2. When the writer combines and MC and a Protag – you get a HERO.
3. The Antagonist always embodies the opposing view and actively sabotages the MC or the Hero. There is no splitting of POV and counteractions. Sometimes there are Henchmen involved for some of the counteraction stuff but they only act on the authority of the Antagonist or at least his implied blessing.

Die Hard movies: McLane (Bruce Willis) is a Hero. We experience the action through his POV and he drives all the rescue efforts

Twilight: MC – Bella; Protag – Edward

Limitless: Eddie Morra (Brad Cooper) – Hero

Nanny McPhee and the Big Bang: the young mother, played by Maggie Gyllenhaal is the MC, while Nanny McPhee (Emma Thompson) is the Protag

The Tourist: MC – Elize; Protag – Alexander/Frank

The Client: MC – Reggie Love (Susan Sarandon); Protag – Mark Sway (Brad Renfro)

Rainman: Charlie Babbitt (Tom Cruise) – Hero
In most of his movies (all of the Mission Impossible; The Last Samurai… any action movie really), Tom Cruise tends to lean towards Hero-type characters. Not so with Knight and Day: MC – June (Cameron Diaz); Protag – John Milner (Tom Cruise)

The Silence of the Lambs: Clarice Starling (Jodie Foster) – Hero

Kill Bill: The Bride (Uma Thurman) – Hero

So next time you wonder who’s the MC, just ask yourself, whose eyes do you see the story through? If they also drive key events with their decisions/actions, that makes them not just the MC but also the Hero. Aileen Wuornos (Charlize Theron) is the Hero in Monster because she both narrates her own story and does all the killing and evading. Doesn’t matter that she’s not a likable character, we identify with her and empathize with her pain and regret when she kills her last victim even though she doesn’t want to. The writer has achieved a perfect blend for the audience of both POV and emotions.

That’s all she wrote…

It sounds strange but I think “the titanic” is the main character.
It sounds strange but if we look at it from a theoretical point of view:

The good guy vs the bad guy : The titanic vs the icebergs.
The main character who changed the most from the start of the film until the end: Titanic. She started out as the mighty and powerful destroyable ship and at the end she “died”.