I've read a good handful from every genre out there, and while I'm working on a fantasy novel, I've considered dipping a toe in the action-spy-espionage genre. Have actual authors done this, and has it hurt their career?
At the moment, I am pitching a historical novel, and at the same time working on a crime novel. It seems to be the practice now for published writers who are established in one genre to use a pen name when publishing in a different genre. If you haven't published anything, yet (like me), I suggest you not worry about it. After all, maybe your action-spy-espionage idea will work better than your fantasy novel; maybe my crime novel will get a better reception than my historical.
Yep, lots of successful authors have done this. I can't think of any examples where it hurt their career, and I can't see why it would - if the new genre was a flop, they could just go back to their old one. Their old-genre fans aren't going to care that they wrote a bad new-genre book. I can think of a few where it was great for their careers - Tess Gerritsen's move from sci-fi to medical thrillers, and Ken Follett moving from spy novels to historical. Not-as-successful authors are a different matter. The problem is marketing. To promote yourself in two genres is essentially marketing two separate authors, which takes double the time and double the budget (if applicable). It's wiser to establish yourself in one genre and then branch out when you have some success and a following behind you. It isn't that writing in different genres is inherently harmful, but that the time you put into writing and marketing a second genre would be better spent establishing yourself in a primary genre.
About a 1/3 of the way through a Sci-Fi Sequel, and have short stories pending prog. or finished in General-fiction Horror, and BDSM Erotica. So sure you can write what you want, in whatever genre you want to.
Genre is really just a marketing tool. If you think you can only write in one genre it's because you have been pigeonholed by publishers and fans. You can write in any predetermined world build you wish. I would rather not write in any genre at all and focus more on making a point as opposed to ensuing escapism. So you can write in any genre you want or lack thereof if you want to. Getting paid is another story, though personally if I were in it only for the money, I don't know why I would be doing it at all.
@BayView The key word was only. Of course one would professionally write to get paid, but if they wrote only to get paid, I can't imagine how substanceless the work would be.
Thinking in genres can be limiting. I don't think "I want to write a medieval drama". It's more of a "oooh, what if there was a king that had an oracle for an adopted sister?" The downside is it can get really annoying trying to describe your work to possible publishers. But I don't think anyone should force themselves to write only one type of story.