I wrote this fic while listening to this song. You should listen to it while reading the fic! :)
You can find the lyrics in both Italian and English here

interchangeable

Hey, do you want to tell me
Of when you had a completely different face
And you were walking toward her
You weren't thinking
That from that moment
You would have been what you are (now)

The church is huge and crowded. I see nothing but happy faces and flowers. Too many flowers. White and red, they're everywhere. Soft and candid cloths of silk garnish the pews and everything looks pure and innocent – sacred, in some ways. Your mother repeated it all the time after all, didn't she? This wedding is a bless from God. Well, if she is sure of it...
I sigh, trying to calm down my thoughts. I promised not to be a jerk or to think spitefully of anyone. It's no one's fault if things ended up the way they did.
Getting a hold of myself, I turn around as the pipe organ starts playing.
The huge, wooden doors open and there you are, holding your mother's arm. You're stunning, wrapped in a plain suit. She, your bride, is right behind you with her father, and she's wearing a long and shining white dress.
Your mother is smiling proudly as you both walk down the nave, while you just show a small and polite smile. I hear people mutter how nervous you must be, because it's written all over your face. I hardly restrain myself from scowling.
If only they knew the true behind your face.

And to think of how all those kisses betrayed
How they stole from our lips the words
(That) we would have wanted to shout
And, so, explain (to me) the meaning of our kisses
And if you can find a meaning
Tell me at least what it is

It's already dark, not only outside, but also here, where we are now. The lights and the heating are both turned off. Even if the lights were on, I wouldn't see a thing, because my eyes are closed. And even though the heating is off, I am not cold. My body is warm, both inside and outside – because I'm wrapped tightly in your arms, trapped between your chest and the wall, and you're kissing me like you've never done before. You're desperate, I can feel it – because I'm drowning in a feeling of despair, too.
I missed you so much I'd love to say it out loud, but your lips are glued to mine, and I'm not complaining about it. Your hands are gently my back, my waist and at the same time you firmly hold my hips. My fingers are tangled in your black, short hair. Everytime our lips meet, my legs shake in an uncontrollable way, the warm feeling in my chest grows bigger and bigger and it makes me literally melt into your arms. I'm afraid my heart is going to explode, because this reminds me of past times – forbidden times – and I almost end up sobbing at the thought that now, you are here, holding me tightly.
Your left hand gently touches my neck, and I feel it. It's cold and thin against my skin.
Your wedding ring.
I immediately push you away – even though it takes all my willpower to let you go.
You look at me, and you realize what we just did. Your arms leave my body, just as suddenly as they grabbed it a few minutes ago.
I let my eyes dig into yours. My breath is irregular – yours too, I can feel it on my face.
“What does this mean?” I ask, swallowing. My voice is a weak and shaky whisper in the darkness.
Your jaw drops, but nothing escapes your lips.
And right when I'm starting to think that you couldn't hurt me more, you speak faintly.
“I don't know.”

You are, you are what I see
A reflection that doesn't belong to me
I don't recognize myself
You, mirror, can you at least tell me who you are?
You are what I see
On (my) mouth I taste your kisses
In (my) eyes I see your eyes
And I finally understand who you are

It's three in the morning, but I can't sleep.
I'm standing in the bathroom, studying my face reflected in the mirror in front of me and illuminated by the light radiated from the abat-jour.
I'm ghostly pale, I lost weight in the last few weeks.
I see you every single day, but I can't approach you anymore. Suddenly, I can't talk to you or look at you – God, even being the same room with you is asphyxiating.
I look at myself and I feel like suffocating.
You kissed me. And I kissed you back. I didn't push you away. I can still feel your lips on mine, your warm hands – your hands that are so big compared to mine – left an invisibile mark on my body.
What we did was right in the past – well, partially right due to the society we live in – but it is totally wrong, now.
I don't want to be a secret lover. I don't want to kiss you in dark rooms, protected from indiscreet eyes. I want to show my love for you to the whole world, proudly saying “Yes, I love this man”. I don't want to hide, crouch in remote corners.
You're the forbidden apple the snake gave to Eva.
You're Eurydice and I'm Orpheus
You're Helen of Troy and I'm Paris.
You're Francesca and I'm Paolo.
 

Except for the fact that you are Siwon, and I'm Jongwoon.

And to think of when, betrayed by all those kisses,
I didn't realize how I was stuck in (your) eyes
On (the) face, in the air there's something about you
And I understood that if I look at my face
I don't recognize myself
But then a nice smile cuts my face
And I tell myself
I'm just like you

It has already been a whole year.
It's late, almost nine in the evening, and she's outside, waiting for you.
Right before leaving, you look at me, apologetically. I pretend to go on with my work. Because this is what we do, now: we pretend, a lot.
We pretend that kiss never happened.
We pretend it wasn't the beginning of an undercover story.
We pretend you don't hold my hand under the table during our lunch break, or when we're both in the elevator.
We pretend we have to stay at the office after hours.
You pretend to love her. Or maybe you do love her – only a bit, deep inside your soul.
And I pretend not to be hurt by the kiss you are sharing, down in the street, in front of everyone's eyes. People smile, looking at you two lovebirds.
You open and close the car door for her and as soon as you get in, you start driving – supposedly heading to a restaurant. Tonight is your night. The night of your love story.
I look at my own face, mirrored in the window of our office, and I flash a smile that disguise my bitterness.
We will never be like you two.
We will keep hiding in dark rooms and elevators.
 

I'm the forbidden apple the snake gave to Eva.
I'm Eurydice and you're Orpheus
I'm Helen of Troy and you're Paris.
I'm Francesca and you're Paolo.

Except for the fact that I am Jongwoon, and you're Siwon.

Orpheus and Eurydice (mentioned in the myth of the same name)
Paris and Helen (mentioned in the Little Iliad by Homer)
Paolo and Francesca (mentioned in The Divine Comedy by Dante Alighieri)

They're all kind of really tragic love stories.
 

A/N: Hi guys~ Just a few words because it's almost midnight and there's school tomorrow, so... I hope you liked this fic, let me know what you think with a comment :) Sorry for any mistakes/typos.
Also, did you like the song? I LOVE IT, it inspired the whole fic and I wanted to share it with you. What do you think of it? :)
Let me know and thanks for reading!
Kisses,
Lara