Hollywood likes to put you in a box – the action writer, the biopic guy, the rom-com chick, etc. But this is not such a horrible thing. Let's just be honest for a second. If you were interviewing applicants for a specific job, and when you asked a particular candidate what he or she was best at, imagine this reply: "I'm great at everything." I bet you most likely would NOT hire that individual.
It's absolutely okay not to be great at everything; if fact, there really are very few "Jack-Of-All-Trades" out there. Employers – including those who hire screenwriters – want to know that they are getting the "best of the best" when they hire. As a result, unless you're already an established and prominent name in the industry, it makes a whole lot of sense to walk willingly into that box - embrace it! Because if you do, you immediately make yourself more employable.
So you've written a screwball comedy or a sci-fi action flick or whatever. Inevitably, assuming your script really is good, almost always you'll hear from the powers that be (producers, agents, managers) something like this: "Great writing. I'm not interested in this movie, but show me what else you've got?" You better have a "something else", and it helps exponentially to have something in the same wheelhouse.
The reality is that almost all American movies – and that's about 2500 each year – are not from original spec scripts. They are on assignment contract projects.
Here's a typical scenario. A major studio has a romantic comedy they've developed with a director and A-list talent already attached. Imagine a Matthew McConaughey/Kate Hudson duo, both have already appeared together in two successful rom-coms: How To Lose A Guy in 10 Day and Fools Gold. So the studio anticipates a hit in the making, but of course they have no script, and they are not interested in experimenting with new or undiscovered writing talent. Can you imagine your agent sending in your underwater CGI action/adventure summer blockbuster spec to try to get you a meeting for the job. Hell no! The studio wants to see the best rom-com writers out there – plain and simple.
So be pragmatic and write the kinds of movies you love to watch and write lots of them. Keep honing your skills and navigate yourself as a premier player in that genre. If you're making money doing something you love, so what if you're the go-to horror guy or the biopic guru? Sure, you want to tell lots of different kinds of stories, but that will come later... when you have a handful of credits, you'll have more access and credibility to pick and choose. But for now, being in the Hollywood Box is one of the safest places to be.