How can we judge films based on our favourite fiction;  is it ever possible to view the film as a stand-alone project?

Like many other people I often wonder why the physical features of characters in novels are ignored when the actors are chosen for the film of the book.   I wrote about The Enchanted April  on Monday;  fair ladies from this story turned into dark haired beauties as they hit the screen.  Did I mind?  No,  because I saw the film first and read the book afterwards and I was captivated by both of them.  It is when they release a film based on a book I know and love that I struggle to accept changes in build or a different hair colour for the main characters.  If you have a mental picture of a fair, slender young woman do you want to watch a curvaceous brunette say her words?  I am still working towards the publication of my first novel but I have already  chosen the actress I would like to play  the main character Ellie, not because I dream of fame and film rights but because it has helped me to write about her.