Okay, so its every author’s dream to have their book made into a movie, and yeah sometimes it can end up a nightmare! But I truly believe that casting the characters that run amok in your head, whilst scribbling down your manuscript, can really help to round them out and keep them straighter in your mind’s eye.
Pinterest is the best tool for this. Just about every actor on the planet is on there and you can create a private board to cast your book (if you feel the need, you can open the board to the public as an extra promotional tool when your book comes out) I found, by doing this, you can create a wonderful linear visual for yourself whilst writing about your characters.
There is, however, a slight pitfall – try not to type cast! Yes, although these guys are purely in your head (for now) type casting a certain actor/ actress into a role can leave your characters as diluted versions of other more famous ones. For example, casting Jennifer Lawrence as a serious, yet plucky arrow slinging hero could land you in a Hunger Games homage – however the same character cast as Lucy Lui could open up a whole new world.
Also, don’t be swayed by actors who tend to play the same characters over and over again. Not mentioning any names, but there are those that are basically themselves just in various outfits – unless you are basing the character on the actor in question, try to stay clear. The actor you choose is purely as a visual, the traits and personality of your character are theirs alone and only serve your story and plot – not the other way round.
If you have a lot of characters in your book, it can help to either define them or boil them down to a concentrated few. If you find that your have a character that does very little to move the plot forward, could their role be simply added to someone else’s? By casting your characters this can be almost instantly identified, if your famous leading lady is merely a wall flower in the book, maybe she needs to be plucked out?
You’ll also find that the ‘who would you cast as your leads’ is a stock question in most interviews – I know I use it! So it’s good to be able to answer this pretty quickly without spending weeks on trying to fit a character to a particular actor.
Great characters are part of the foundation of a great book. But amazing characters can make a series. If your readers are hungry to know what happens to them after the book ends, well that’s the sign that, as an author, you’ve down something right. So cast away fellow writers!
Nicky Peacock - YA & Horror Author 
I always think it helps to have a photo of your character, not necessarily an actor, but someone you know or have known once.