After giving a bit more thought, I realised that the real question wasn't whether or not I planned everything before I start writing, but more whether everything goes according to plan when I do start writing.
If I answered 'yes' you'd probably not believe me. Especially if you're a writer yourself. So how much planning do you do and how much do you go with the flow?
Myself? I'm a bit of both. When I started writing twenty years ago, I was of the opinion that you should let the story tell itself. Needless to say, I struggled to write. I'd start off all fired up, but would run out of fuel very quickly because I had no plan of where the story was going.
Fortunately, with practice (and age) comes wisdom. I started planning my novels. Not in much detail, just a few points on where I wanted the story to go: where it should start, where it should be around the middle and where it should end. This gave me some goals to work (write) towards. This was a good start, but again I found that I ran out of creative juices because the middle and end came too close to the start.
A little more planning followed and I soon found myself planning my novels in a lot more detail. Take my second sci-fi novel, The Fourth Portal, for example. I started by planning the main threads of the story: the Explorer crashing on the planet and the government on Selenta VI trying to find out what happened. Once I had my two main storylines planned, I started working on all the sub-plots; each character's own story and how it should tie into the bigger plot.
As any writer will tell you, things don't always work out the way you planned. When I originally wrote Family Secrets for radio, I had the storyline planned when I started writing. Then the unexpected happened. I was writing a particular scene where one of the characters stumbled across a secret from the family's past. I originally intended for a new character to be the catalyst for this revelation. But when the time came, my characters took on a life of their own, resulting in an entirely different secret being revealed by a character for whom I had something completely different planned. Some would say that I should have gone back and rewriten the scene to fit in with what I originally planned. Instead, I decided to let the change stay in the story. It gave that particular character more depth. This change in plans also resulted in three new storylines which were never part of what I originally planned, giving me enough plot for a second and third season of the series.
Over the years I've learned that before you can start writing, you have to know what your story is about. Then you have to decide where it's going to start, where it's going to end and work out where your characters will need to be by the middle of your story to reach the end. Along the way, things may not work out the way you planned. Sometimes it's better to rewrite and stick with your plan. Other times things work out better when the unexpected happens.
So to come back to my friends original question. Do I plan every little detail of my novels before I start writing? Yes, but I'm prepared to re-evaluate my plan if things change along the way.l