The main characters in the book have defined ages - Eric: late twenties, Scout: around 21.
Some of the actors suggested here are way too old to play the roles.

James McAvoy / David Tennant would be a good choices for Eric
Michael Caine / John Hurt as Dr Fidorous sounds great
I have no suggestions for Scout / Cleo, but I think that Scout and Cleo should be played by different actresses- would be great if they were very similar in looks (perhaps sisters in real life, with Cleo being played by the older one)