I think you need imagination in order to employ diction, a different sort of imagination than the one you use to generate plot. You know how they say you need to imagine success in order to reach it? That's what I'm referring to here. For me, there are contemporary and retrograde imagination. Neither is difficult to achieve so long as you keep an eye one yourself to avoid straying off course.
Contemporary imagination is that whole "imagine your success" thing. Imagine that you've got a copy of your novel from the future. What details would the future you include in the text? What word flow? How much humor? How much drama? Aim for the near future too. 40-Year-Old Mario or 50-Year-Old Mario will have experiences that I can't possibly imagine right now. On the other hand, 30-Year-Old Mario is just around the corner.
Put another way, your style of writing changes gradually over time. If you've got any lingering scraps from your grade school days, you'll see what I mean. The difference is like night and day. My writing five years ago as an undergrad wasn't as sharp as it is now, and my writing now isn't as sharp as it will be five years in the future. True, some elements of style will still be there after a relatively short amount of time, but you may also drop some bad habits and pick up some new ones.
Retrograde imagination is another matter. With this, I'm putting myself in the shoes of another time period altogether. Right now, in addition to the alien invasion novel, I'm working on a story that spans the 1890s to the 1910s. Not only that, but I'm also writing in the voices of the characters of that time. It's totally anachronistic to have a character walk into a room and say, "Yo, yo! Wassup?" Even people from that time period don't quite speak the same way anymore. Besse Cooper (at 115, she's the oldest living person at the time of this writing) doesn't speak today the same way she did when she was a little girl and William McKinley was assassinated in 1901.
The only way to anticipate and imagine how people of a certain time period might write or speak is to expose yourself to the literature of that time period. If I want to write with a 19th century flavor, I need to read the Mark Twain's, the Joseph Conrad's, and the Mary Shelley's. If I want to write in a very BC fashion, I need to read Homer and Virgil. Like contemporary imagination, you're trying to tune your mental ear to a voice; however, it's not precisely your voice. If I were to write a direct sequel to The Invisible Man, I would read through the original, and then, as I'm writing, I would ask myself, "How would H.G. Wells write this?" And then I would ask what I would do as a writer.
The goal here is to try and stay true to yourself while fitting into a certain mold. I suppose actors doing period films run into the same obstacles.