When you see something on film it is very rarely how you first imagined it to be when reading a book. However, once you have seen something on film, the memory you originally had in your mind tends to be erased and replaced with the film version. Now when I imagine characters from LotR, it is the actors faces which I see, not my original impressions.
Saying that, the film versions of Gandalf, Aragorn, Frodo and Gimli were all remarkably close to how I had always imagined them. Legolas, Faramir, Eomer and Theoden were quite different (and in particular chubby-elf Haldir).