Anne Heche's character is actually lying. SPOILERS
Seeing the film a second time, I am absolutely struck that nobody has pointed this out just yet:
The film is not as much about identity as it also is about truth and lies.
We, as an audience, just as Anna's family, don't accept that Sean IS indeed Sean. Even if we believe in re-incarnation, in the context of the movie we simply don't. It's the same with his family, except for one: Anna.
Now, stay with me for a second. We all know the letter scenes, the photos, but... I must say, I find Anne Heche's character a lot more questionable than Sean. While Sean has found the letters, even after it is revealed he has, it is never discussed why he followed Heche's character, or why he loved Anna. He himself states that he believes to be Sean, that it feels like Deja-Vu. And I don't think he lied.
However, when Heche's character tells him that Sean loves HER, that he wanted to leave his wife, that he give her the letter's Anna sent him, unopened... I really have to question whether or not this is true. Remember that she actually wanted to give Anna the letters as a "wedding gift" - it's actually an act of revenge. She herself does this to proof Anna that Sean loved her! Why would she need to do that if that would have been the truth?
Heche's character seems so bitter to me that IF Sean would actually be Sean, then that could potentially ruin HER life, HER truth she's been living all these years. And it's not that the knowledge of Sean finding the letters made him consider the he was wrong, or come out as a liar. It's that he is convinced that Heche is telling the truth, that Sean loved HER and not Anna... yet Heche has no proof whatsoever, except for her possession of the letters.
But what if she stole the letters? What if Sean only gave them to her to shut her up?
The truth is: we accept Heche's statements without any explanation just as Kidman accepts Sean's "confession" that he isn't Sean, without any proof or explanation.
Seeing it a second time... in the reality of the film, I think Sean never lied. I think he IS SEAN. He is so convinced of it, that the only counterproof is that Heche's character claims he loved her... without any proof whatsoever.
The film thus isn't all about identity - it's about the lies we actually believe to make our lives easier, to actually "go on living". Anna knew how hard her life would be IF Sean would be Sean. So she accepts the first answer of "No, I am not Sean." - without any further proof or explanation. She wants to accept it, just as we do, just as Anne Heche's character wants to accept that Sean loved her, just as Sean wants to accept that he is insane.
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