These two disciplines promote different aspects of fiction – experience lends itself to realism, imagination to romanticism (in the broadest sense of the word). The writer therefore has a heady cocktail of material that he carries right along with him wherever he goes.
It makes sense that if you augment these two reservoirs then you will naturally have a broader set of experiences and ideas with which to inform your writing – theoretically your work will be more believable via experience and more creative via imagination. These two disciplines require different means to extend them – experience needs feeding, and imagination needs exercising.
So a fastidious writer will spend his time day-dreaming while gobbling up experience like a starving man, right?
Obviously there are limits, and a wealth of experience and a fertile and well flexed imagination are only a part of being a writer – but there is a certain truth to this.
Which is why I think writers are drawn to the dark a little – preferring a tempest to a heat-wave; raking over past experiences to reinterpret and understand; embracing melancholy, longing, dissatisfaction (as well as happiness) to better feel and understand it; quizzing people with genuine interest about painful things that have happened to them, making one speculative remark too many at dinner-party conversations.
There is only so far you can go with experiencing the darker side of human nature without getting arrested, but with the imagination there are no limits – and this is where a writer will walk themselves into some pretty dark places.
Sounds grim, right? But as writers I think we are professionally obliged to do it – that's our job – to walk on the darkside (either for real or in our imagination, or both) and write about it - and hopefully by doing so we will say something that speaks to people about their everyday lives.