It's best not to have the possible film adaptation of your book in your mind as you write your characters for the first time - but it is a good idea not to make your plots or location far too complex, as that would make for a largely unfilm-able book.
In this day and age, we are all aware that what we write could be possibly mooted for a film screenplay or a TV/theatre script - but don't let that exciting thought ever interrupt the writing process, as you must never stem the flow of your words, by thinking of filming locations or which actor would play your protagonist.
I have in mind here, The Martian, which was brought to life beautifully as a film and was excellently cast too.
Andy Weir, the author of the book, at first self-published The Martian, and it was later picked up by a mainstream publisher. If you are a fan of the book/ film - have a look at this great Wikipedia link.
If you feel that the book you are writing could be turned into a movie or a TV film/series, carry on with writing your first couple of drafts, but then when you get onto the third and fourth draft, read the manuscript as if you were a film producer.
Making Sense: Ask yourself, is there any part of your book that would not make sense to a cinema audience? If so, now is the time to make the change/s.
One of my favourite books is called: Go See The Movie In Your Head (Joseph E. Shorr) - it's a psychology book but the title reminded me that even if your book is not turned into a film - at least you will have weeded out any elements that do not encourage the readers' imagination - which always rules supreme.
Happy drafting - and I hope to see the movie of your book at the cinema in the not too distant future!