How do the cast of the films compare to the characters in your head?  

Apart from the characters,I don't like the Harry Potter films since it seems to be a fast forward of the books.If one doesn't read the books,he/she would not be able to understand the film.
And for the characters,Lord Voldemort is perfectly good,and as one sees the film,character imagination comes automatically..

One thing the potter movies boast of having achieved is the exact picturisation of the characters and most of the scenes and of course not all of them. I don think there could be any substitutes for the characters Hagrid, Snape, Dumbledore and more importantly the Weasely twins in the first three parts. I feel most of the characters are well chosen. I read in an interview that even Rowling is found to be satisfied with the cast in the film. One thing the directors failed to concentrate on is the story on the whole. I don think anyone who has not read the book will be able to understand the movies and I reckon everyone of them will leave the theatre with their minds full of doubts. Its not the directors to be blamed of as they do have the time constraints and they will have to show a four hundred page book in 150 minutes. I think they have done their best.

i totally agree with you.. alan rickman is perfect for the role of snape.. also prefer Richard Harris as Albus Dumbledore (Philosopher's Stone and Chamber of Secrets). and of course Maggie Smith as Minerva McGonagall (cant wait to watch her on the hogwarts battle in the next movie)

Most of the characters fit what was going on in my head pretty good. Some are perfect. Like Luna, Hagrid, Voldemort. Snape too, kinda.

Although I think Alan Rickman makes a perfect Snape, and I would not want anyone else to fill that roll, I pictured him different when it comes to looks. I thought of him as skinnier and with sharper, more severe facial features. And with a goatee and longer hair. At least a little past the shoulders.

Also, Hermione is supposed to be ugly, not pretty. lol. I think they should have at least kept her hair bushy, like they did in the first film. She was supposed to go through a transformation for the yule ball, and then decides that she likes who she is and spending all that times on looks is pointless for her. But in the movie she looked exactly the same as she always does. They ruined it.

Yes. the choices were perfect. I love all the characters. They couldnt have chosen any better actors for the given roles. No matter how ill plotted the movies are, i Love them allllllllll

Harry Potter film, i feel, was very well written, but it is not possible to read all the Potter books written, Film is perfect, for those who do not offer to read it due to certain other immportant items to be attended for. what ever it may be it is perfect for the charectors,actors and other plots in the film. a person with open mind, willing to know at actually going on, in the Potter literature is safely see the picture and relates.