To be fair, I haven't read The Trial in a few years. But I think the film actually follows the text quite closely. I noticed a few omissions and changes, but not many. One interesting difference is that the parable, Before the Law, serves as a preamble to the film. And is followed by a short introduction where Welles talks about the text itself and the ways in which it's been interpreted. Sounds cheesy, but it works well. The plot of the novel really isn't that gripping, but somehow, Welles manages to retain your interest. Actually, K. as a character is somehow more emotionally affecting than he is in the text. I think this is probably because the third person narrator of the novel has been replaced by the camera, a cross-over that I generally find fascinating and difficult and am still thinking through.
Anyhow, what's most brilliant about the movie is the general aesthetic. Visually, it's genius. It's the kind of thing where I was like, "my god, that's exactly what it would look like! it's what I was envisioning all along, and I didn't even realize!" This is all the more impressive because Kafka isn't really a scene-painter in his works - it's fairly conceptual stuff, with very sparse descriptions. And yet, Welles' portrayal strikes one as being exactly right.
Furthermore, as an adaptation, it's of course also an interpretation, and what I found fascinating was the way that it visually represented the sort of "rise of bureaucracy, terrors of capitalism" interpretation in a truly compelling way. Granted, I may be more prone to this at the moment, given that I'm TAing a class on Marxist theory, but I've always found that reading of Kafka somewhat overdone, and here, for once, I was truly convinced.
The final scene sort of threw me. There's a change to the text that's executed in a somewhat pointed way, and I'm still not sure what to make of it. I don't mind taking liberties with the work, but if it's meant to be signpost to the text's resonance with current (well, in the 1960s) conditions, I'm somewhat unimpressed.
Still, ultimately, an excellent film. Worth investigating, especially if you're familiar with the book. It's really a phenomenal tribute to Kafka's work.