Specs, or scripts written on speculation, are a screenwriter's calling card these days, especially for TV. I have a number of spec feature scripts that I've written, hoping to sell or get financed. But I recently finished writing my first spec TV script, an episode of the ABC mystery series, "Castle." My agent can now use this in order to get me meetings, hopefully, for TV writing jobs.
So, being a fan of Castle, and hoping that there aren't a lot of spec Castle scripts floating around, I decided to write one. For one-hour dramas, Dexter is probably the most popular spec to write, (which is why I didn't - too many comparisons might be leading to reader fatigue), and the idea I had was more in line with a Castle anyway. I suspect that in that genre, The Mentalist and Castle are two of the best shows to write specs for, but who knows? Besides, not working on the show, it's impossible to know it better than those who do. That's what makes writing specs tricky.
It's unlikely that a producer on Castle will read my script. In fact, I hope they don't because there is no way it can be as close to a Castle script as one written in house. It will only annoy a Castle producer. However, producers who work on other shows will be familiar with Castle but not from the inside out. And they will hopefully take to my script as being a good Castle - and therefore, I might be someone who can buy into their own show as well.
If you want to write a spec, it helps if you use Final Draft or Screenwriter, as they both come with templates of shows, (though neither has Castle.) Also, make sure to READ scripts of the show you want to write a spec for. And make sure they are scripts, not transcripts.
TV screenwriting is not about personal voice so much as being able to execute someone else's vision. The individual writer's goal is to come up with a story that works for the series and then execute the characters consistently with the show. I think I've done that enough to pass muster by anyone except perhaps the showrunner for Castle, and now my agent has another sample of my work with which to get me up for paying work.