Casting

I saw the film and enjoyed it - more than I thought I would to be honest. I thought Emilia Clark and Jenna Coleman were both good and fitted their roles but I also thought either one could have played either part.
Now that I'm reading the book I'm inclined to think Jenna Coleman might have fitted Lou as described in the book just a little bit more closely.
It doesn't really matter to my enjoyment of the film and I suspect it's only an accident of timing and career arcs who got which role.
Perhaps in an alternative universe the roles were reversed - and a younger Benedict Cumberbatch was cast as Will Traynor?