Which genre any writer chooses to operate in is dependent on the genre they enjoy. I write predominately in the mystery/detective genre because I am such a big fan of Robert B. Parker’s Spenser and Jessie Stone, Janet Evanovich’s Stephanie Plum, and Raymond Chandler’s Philip Marlowe. But it is a great exercise for the good writer to able to write across genres even though he may never have the opportunity to use this versatility. Many stories have the ability to cross or mix genres, so that a historical romance may have elements of the thriller within its plot and the science fiction novel may be a detective story.
What we're really striving for is versatility. It benefits the writer to develop an across the board range of skills to ensure that when your material has to go into strange lands it doesn't stick out like a sore thumb. This ability to blend other forms into your own will serve you well, because readers crave variety in what they read. If the average reader is anything like me, they may not pick up anything outside their favorite style. But, if there is an element of romance in my mystery novel, I’m remarkably fine with it.
One of the easiest ways to play with cross genre writing, especially if you are trying something new, is to consider the pacing of the story. By this I mean the pace of the entire work, including narrative and dialog. A love story is generally languid whereas the detective novel is much more likely to be punchy with short sentences and snappy dialog. By varying the pacing in accordance with your story elements, you may be able blend historical romance with science fiction, or elements of comedy into your thriller.
Non-writers are mostly startled to discover that you can shift their emotions merely be varying the length of the sentences you write. But a succession of short, sharp sentences will get the adrenalin flowing whereas lengthy descriptive passages will relax and calm. In that sense the writer is a magician, directing the attention of the audience words. He can lift them with rousing deeds or depress them with tragedy; arouse them with passion or anger them with injustice and, if he is good, he can do them all.