What is the likelihood of my LDS novel being made into a movie? Is there any possibility at all? Would I be asked to write the screenplay? Would the movie possibly get national release or would it be a "Utah" movie? Is this something I should be thinking about while I'm writing my novel? Should I create scenes, etc. that would look good on the big screen? And assuming my book was made into a movie, does the author have any control over who plays the different characters?
Oh, geez. First off, finish your novel. This isn't even a conversation we should be having until after the book is published and has a phenomenal sales record (as in, hundreds of thousands of copies sold in a very short period of time). While writing, concentrate on creating scenes and plots that are good for the story. Don't cheat your readers by throwing in something that doesn't belong just because it might look good in the movie.
Now, as to possibility: yes, it's possible that an LDS novel can be made into a movie for national release. Work and the Glory. The Other Side of Heaven. It is also possible that a movie could be made by an LDS production company. Almost every year at the LDSBA, someone stops by my booth to ask if I have any books that would do well as a movie. I give them a free copy (because I am an eternal optimist and you never know...) but I never tell the author because most of these companies are out of business by the next LDSBA.
As to probability: slim to none. For a book to be made into a movie by "Hollywood", it either has to be a runaway NYT best seller (but that doesn't guarantee it), or someone in the movie industry has to love it (someone who also has the power, influence, and financing to produce it). Chances are infinitesimally better (slender to none) that it will be picked up by an LDS production company. The LDS movie buying population is just too small to support the cost of making a strictly LDS movie (except for those based on the scriptures, for example, the Liken series or those produced by the Church).
Assuming the Red Sea parts and you find yourself with an offer to produce a movie based on your book: No, you would not be asked to write the screenplay. No, you would not have any control over who the actors are. You would also have no control over the title, the director, which scenes from the book are included, which are deleted or which are totally changed to the point that your original story is barely recognizable. Yes, some authors do have control over these things written into their contract, but for 99.999...% of us the only control we have is whether or not to accept or reject whatever the production company offers us.