It's Not Just the Writing
When I was a teenager and first started writing with the goal of publication, I thought that a published writer's career meant one thing: you sit and you write. That's it. That's a writer's job, isn't it? What else would a writer possibly have to do? What other skill would a writer possibly need?
Turns out (as with many of my teenage assumptions), I was completely wrong. The modern day writer needs, in addition to the skill of writing, a whole host of other skills that I never really associated with the career. Like:
The ability to be a socialite
This is a big one. My friends and I occasionally joke that a writer's life is composed of extremes. Most of the time, you are an Uber Introvert and spend large swaths of time being hermity in front of a computer. But every now and then, you'll be immersed in extrovert activities—okay, not just extrovert activities, but ZOMGEXTROVERTACTIVITIESZOMG. You will go on tour, where you will talk nonstop to large groups of people every single day from early morning to late night for weeks on end. You will attend conventions and conferences, where (again) from morning until night, you'll need to be able to mingle, network, greet, do interviews and panels, speak in front of huge audiences, and party with strangers and friends and acquaintances. As a modern day writer, no matter how introverted you are, you need to learn to socialize on a large scale. This can all be really fun, of course, but it's also scary for those who are inherently shy. (Like me, and seemingly the vast majority of writers.)
The ability to say 'No'
Saying 'No' can be extremely difficult to do, especially when as a writer you are grateful and generous to those who want to learn more about you and your book as well as more about the writing and publishing process. Bloggers and schoolkids will want to interview you. Educators will ask you to visit. Other writers who need blurbs may come to you with their requests. These are the hardest to turn down, because we all remember being that debut author who needed blurbs for her very first book. But eventually you'll realize that if you hope to find any time for your own writing (and, you know, your 'down time', whatever that is), you will have to turn down some of those interview/blurb/visit requests. Pick out the books you love most for blurbing. Have a quota for how many interviews/visits you agree to each month. Sometimes your agent/publisher will help you out by putting their feet down for you when you can't bring yourself to. :) This skill, I think, may be the absolute hardest one to learn. Most writers are gracious, and hate to disappoint. But I'm sure everyone would prefer a new book from you instead!
The ability to be a hermit
On the flip side of the "being a socialite" skill, writers need to figure out how to keep the hermity parts of their lifestyle intact as well. Writing, by its nature, is a solitary profession, and unless you have a co-author working with you, you need to be alone during your writing time, undisturbed and immersed in your fictional world. With all of the noise and buzz that comes with being a professional writer, you will need to learn to tune it all out and return to that peaceful place where you can concentrate on nothing but your words.
The ability to manage money
Ah, money! Being a full-time, professional writer means learning how to manage your money in ways that regular 9-to-5ers usually don't need to do. Writers get paid irregularly and in weird spurts, and you can never really tell when your next check will arrive. On top of that, taxes are much more complicated as a self-employed artist. You don't work for a corporation that helps you deduct taxes automatically from your paychecks. You will need to learn how to pay taxes quarterly, how to set up your own health insurance and retirement funds, how to write things off, how to adapt to such an irregular payment schedule, and, if you choose to set up a corporation in order to save on taxes, how to manage your business. Writers often turn professional not knowing how unusual a writer's payment/tax world can be. Educate yourself in this field. (I recommend getting a CPA! Don't try to TurboTax this by yourself unless you're seriously good at taxes. It's worth the money.)
Of course, at the end of the day, the most important skill you need is still being able to write books. Writers write. All of the above is secondary. (ok, except maybe the hermity part...and the money management...)
Are you a published author? If so, what skills did you find yourself picking up that you didn't expect to?