I think the film that most effectively employs actors in adapting an original work is The Innocents, directed in 1961 by Jack Clayton and based on Henry James's 1898 novella, The Turn of the Screw. Although there are only a handful of actors in this film, I think this works to the film's advantage in conveying the message of the novel.
This is a very intimate film. Because of the lack of scenery changes and the minimalist set, I feel as though I experience this story right along with the characters. The home and the grounds feel so real, they do not feel Hollywood. I think this helps make the characters feel real too. I think the children, Miles (Martin Stephens) and Flora (Pamela Franklin), are amazing, and they really capture the sneakiness that seems to be present in the characters. I also think they do an excellent job of acting too old for their age.
Another character I think is amazing is the governess, portrayed by Deborah Kerr. I really feel the sense of desperation she feels to save the children. It is also evident how she becomes increasingly paranoid throughout the film. The emotions I feel by simply watching people, such as Kerr, act with a lack of sets, extras, and special effects are amazing.
It seems as though it would be difficult to make a horror film, but this is one of the best I have every seen. The casting in this film is perfect as far as capturing the emotion of the novel. The characters are played out perfectly, to the point that I feel I am watching a real scene unfold rather than a performance.