The Sacred Space of the Originals

I often think I like a movie better if it follows its source material faithfully.

But I might be looking through silver-colored glasses. In revisiting The Lord of the Rings: The Return of the King last night on TV, I noticed that I still enjoy this film immensely, despite the tweaks to much of the dialogue found in J.R.R. Tolkien's book. There's at least one precedent: I also love David Lean's brilliant Great Expectations, though the movie's end, in which Pip tears away the curtains hiding Estella from the world, is nothing like the novel's finale.

So why am I fine with the changes in these adaptations...yet horrified by the ones in Garth Jennings' The Hitchhiker's Guide to the Galaxy?

I know--I'm comparing two masterpieces to a misfire. It's not fair. But I'm trying to find a pattern, and the only one I can see is a significant difference in quality, resulting from better direction, scripts, editing, and other elements.

Could that be it, then? Could I like something that, theoretically, could be completely different from its source material if it's just done well?

I'm not going to discount that. I'll have to mull it for a while. Maybe I'm not the purist I think I am. Or maybe I'm a purist just with movies.

I know that if it's good, I'll like it. Maybe I'll just go by that.