The following comment from the writer Frank Cappello is taken from this YouTube video about the movie. The comment has very few likes, so I thought its a good idea to share it more.

'Hey I wrote this thing!   I love how they explore a film and never mention the writing.  So I guess I'll tell the story because it's got some interesting twists and turns.  Maybe the fans will like hearing how it came about.    

I got the assignment to rewrite a Constantine script that Warner Brothers had spent two plus years developing.  The original writer Brodbin pulled his script from two of the Hellblazer comics, Original Sins and Dangerous Habits.  By the time I came on in 2000 the script was several drafts and two writers past Brodbin.  It was a cut and paste mess, the tone was something like Indiana Jones in Chinatown, full of Swartzenegger one liners and lots of gun play. Even though it would be my first studio writing gig, I thought it was trash - didn't really want to do it - until they sent me some of the graphic novels including the two Brodbin had pulled from.  I was blown away.  This character was a writer's dream - a crass selfish bastard.  But I was really a nobody that happened to have written a script that was a favorite around the industry.  So it would be a long shot for me to get the job.  I had to do something bold.  So I sat down and wrote what I thought would be a worthy opening.  That was 9 pages that included finding the Spear (in a completely different way then the film) and the Exorcist scene using the mirror to suck out the demon from the child.  That got me the job.  My marching orders were to do a polish, make their cut and paste smooth and maybe rework the ending.  I wanted to do good since this was my studio virgin assignment but I just couldn't wrap my head around what they wanted to keep.   To be blunt - it was shit.  So I did what a writer is never supposed to do - I went rouge.  I threw out probably over a million dollars worth of scripts and started over.   I  didn't tell anyone, even my agent.  I made a whole new story - invented the twin sister subplot - threw away the standard pentagram and candles and replaced them with a cat and a bucket of water.   I spent two months getting my vastly different version together and then turned it in - thinking "they're gonna love this!"  And the execs hated it.  How could I have the audacity to throw out 3 writers work without asking??   I was out.  The career was over before it began.  I thought I might not even get paid my balance.  Then something happened that only happens to starlets in Hollywood cafes:  Two months after turning it in, the president of Warners,  Lorenzo Bonaventura,   happened to pick up my script and read it that weekend. On Monday he came in and he told the execs that had stopped talking to me - "Now THIS is a movie."  I would go on to write almost 50 drafts  over 4 years (on and off work) for three different directors and two different stars.  I wrote so many drafts I started calling it Constantpain.  Some of my drafts were labeled by the TIME - not the date.  

One of the last scenes I wrote, again telling no one -  was nixed by one the major producers because he said it was too late to add a new idea.  "But it makes the film better!"  No.   So... I went over his head and slipped it to one the execs who loved it and it got into the film.  Yeah I got some dagger eyes a few weeks after that.  The idea had to do with how John got his ability.  It's not in the comics and I know the fans hated it back then - but I still believed it would solve a plot hole for movie audiences and the idea came straight out of a personal experience.  People often ask writers how they come up with ideas.  Well here goes -- My Dad was building a house and passed out.  They discovered he had blockages in five major arteries so they got him into the hospital and did heart bypass surgery. My sister and my Mom sat in the waiting room and the Doctor came out to tell us  that when they were finished they couldn't restart his heart.  They scrambled with the heart pump to get it restarted and finally did.  But for 4 minutes my father was clinically dead.  Now my dad was a strong man, joined the army when he was 17 and fought in the Korean War - but he never told us what he did in the trenches.  I believe because he may have taken a few lives and since he was raised Catholic he was afraid of dying because he knew exactly where he was going.  (Sound familiar?)  Before the surgery he was a very outgoing man, high in spirits - a prankster at heart.  After the surgery he changed.  He  was quiet - introspective.  Brooding.  After a year of this I finally confronted him - "You've changed - what happened?"  It took him a while to get it out but he said since the surgery he was having the most disgusting, horrific nightmares.  He couldn't even describe what he saw.  It was effecting his sleep. I tried to get him to see a psychiatrist but he was "old school" and thought they were all quacks.   This is what I put into Constantine - his origin.  That he had tried to commit suicide and because it was a mortal sin, went to hell- but then they revived him - he was never the same.  He could see the demons right here and also knew exactly where he was going when he died.  

The film was released and wasn't the success Warners wanted, so the first sequel I was writing in 2005 never got greenlit.  Over the years the film has gathered more fans and many have told me it's one of their favorites.  I'm also surprised that women usually like the film more than men.  I think it has to do with the "balance" between Heaven and Hell right here on Earth.  I got another chance to write a sequel in 2010 when I woke from a dream and quickly wrote 9 pages of what my agent thought would be a great sequel opening.  it got the attention of the producers and the director and when asked - so what's rest of the story?  "We went to hell in the first - we're going to Heaven in this one."  "Why"  To burn it down.'

PS. A sequel is in the works, with Keanu starring in it, Frank isn't involved it seems.