Petunia

There are many self-insert fanfiction where people wake up as eleven years old Harry Potter about to get his Hogwarts letter, a hero in a Marvel movie, or in Twilight as Bella Swan on her way to Forks, the main characters with all their issues and specialties,

But - what if you woke up one day or perhaps after your death, and you find yourself in the world of your favourite children's book, only you aren't the main character? What if instead, you are someone you have a lot more in common with, someone normal and human and, in the grand scheme of things, rather unimportant?

You'd still know the plot the main character will follow, maybe you can give them some tips... On the other hand, if you change things, it might not be for the better despite your best intentions, and the planned happy ending might never come to pass. Could you stand by and let it all happen the way it was meant to? Even if people are hurt and die in the meantime?

What if you wake up one day and you are Petunia Dursley née Evans?

Ordinary, non-magical, plain, middle-aged Petunia.

Well, perhaps not quite middle aged yet - she's a bit older than her sister, and Lily was only in her early twenties when she died, so a couple years later, still pre-canon Harry Potter timeline (though after that Perfectly Normal first chapter, of course), Petunia might only be thirty. You might not have been thirty in a long time, or you might never have been thirty yet, or it's exactly your age range, either way you wake up and find yourself in the extraordinarily ordinary suburb of little winging, married to Vernon Dursley (the horror! No, honestly, what a horror that would be, to wake up in bed next to a man who you have never met and yet who calls you his wife, or worse his 'Pet' and it takes you a while to figure out that's a short form for Petunia not some claim on you as subservient in the relationship. 

Once you realize that you are really in the magical world, that this isn't a dream, or if it is it is a particularly realistic one you just can't wake up from (you never quite shake the fear of being trapped in a coma, but as time goes on it is just something you sometimes think about, like back in your own world after watching the Matrix and realizing you can't prove that you aren't in one, it's not something that rules your life), you grieve for your old life, the relationships lost, the friends and family you will never see again. 

Knowing that you are Petunia Dursley, and the events of the Harry Potter books haven't happened yet, you try to decide what to do with the knowledge you have.

At first, you think it might be best to do nothing - the books have a happy ending, and who knows what your meddling might do to that? But the hope that your presence as Petunia won't change much is a mote point soon, because there is no way in hell you will treat an innocent child the way Petunia had been treating her nephew. 

So, changes will be made.

Still, you are scared to show your knowledge and end up locked up in the department of mysteries or something. You have no magic, no way to defend yourself.

And anyway, you got some years yet to figure it out. 

(Maybe, if you install some self worth in Harry before his first year, he won't go after Voldemort and he'll end up trapped by the mirror of erised and it will all be over before it even started)