Write What You Know

'Write what you know' is one of the most fatuous pieces of advice
ever given to fiction writers, mainly because it propagates the myth
that fiction writing is nothing more than thinly veiled autobiography.
I cannot speak for all writers, of course, but the reason why I write
is to discover as much as I can about things that I don't know about;
to explore the psychology of my characters (none of whom are me) as
they extricate themselves from perilous situations that I hope never to
find myself in. To limit my writing to what I already know seems
tiresome in the extreme; rather like an explorer being told not to go
beyond the boundaries of existing printed maps.

Writers: you should write about just what the hell you want to write
about, irrespective of whether you already 'know' about it or not.
First and foremost, you should use your imagination. The raw materials
that your imagination works upon are naturally going to be your own
experiences, so you are still writing 'what you know' at one remove.
Obviously, if you decide to write in detail concerning something that
you don't already know about, then what you do is go out and find out
everything you need to know. That is what research is for.

A better piece of advice would be 'write what you are passionate
about.' If you are already passionate about a given theme or subject,
then it's almost certain that you already know a good deal about it (so
again you are 'writing what you know' without even being told to). It's
also very likely that your enthusiasm will be communicated to the
readers and they are going to become passionate about it, too.