I have found the last bit particularly useful, because sometimes I have created characters which don't work. Don't get me wrong, they are lovely and I know a lot about them but they come with the wrong sort of emotional baggage.
For example, the character of Valens in The Gladiator's Honour had to be recast because his back story didn't work. I also knew for him that it was probably best if I kept a Russell Crowe type in my mind as he is fairly seared in people's brains as a gladiator.
Sometimes it is a little gesture or the tone of a voice that convinces an actor could play the part. For example, while I was writing The Roman's Virgin Mistress, I could see Christian Bale playing Fortis. He came into my mind, not because of Batman Begins, but because of his voice in Howl's Moving Castle. I wanted that voice. I could also see him reaching out and pulling Silvana to safety. That scene in chapter one was pivotal to me.
The other thing I have recently discovered because I am writing a duet is that if you have a very strong idea about the second hero (in this case Richard Armitage), you need to have equal confidence about the first hero. I had like the look of English polo captain, Henry Brett, and his reputation as a bad boy appeals, BUT I didn't know if he could play the part, and the other hero threatened to take over. Not good. I have had to recast, and now have more of James Purefoy type character in my mind. And the thing feels more evenly balanced.
Does anyone else do this? Consider who might play the hero? Or even change the casting? I know it works for me. And in writing, it is not about what works for someone else but what works for the writer.