I have not read Lipstadt's book. I have read Richard Evans' fascinating Lying About Hitler in which he details the extensive research he did on Irving's writing. Evans was able to show, conclusively, that Irving consistently distorted the historical record to meet his own ideological ends--even going so far as ignoring his own notes, which indicated that he clearly understood the original texts.
I have also read The Holocaust on Trial by D.D. Guttenplan which is an overview of the trial from a journalist's perspective.
Denial clearly strives to remain accurate and therein lies the writing rub. The problem with "true life" dramas is that the writers can either sacrifice accuracy for DRAMA or fall back on making a documentary (there was in fact a docudrama made of the case with John Castle as Irving).
The unbelievably fantastic actors include Tom Wilkinson (Batman Begins) as Richard Rampton, the trial lawyer; Rachel Weisz in glowing form as Deborah Lipstadt (in the "Making Of" clip, Weisz and Lipstadt are interviewed together; an older Lipstadt reports, "My friends say she captured my accent completely!"); Mark Gatiss looking quite unlike his dapper Mycroft self.
Timothy Spall delivers a David Irving who is, based on my reading, closer to Guttenplan's assessment than Evans'. Evans loathed Irving. Guttenplan, in typical journalistic fashion, wanted to understand him as a man of painfully prejudiced and racist views who nevertheless did not appear to carry about any prejudiced and racist visceral responses (such as getting uncomfortable around the Jewish Guttenplan, for example, which Irving didn't). Irving is not a good guy, but he is a sad one. In the trial, he wanted to sell himself as a maverick at the same time that he wanted acceptance by the British "establishment." Spall accomplishes this complexity with some monologues, spot-on body language, and a smattering of back-and-forth dialog.
Alex Jennings is so perfectly cast as Sir Justice Charles Gray, I clapped my hands when I saw him.
What makes the film work in the end, however, is not the cast. They help. Tremendously! And since I love legal/crime stuff so much, I probably would still have enjoyed viewing Denial without the second point, but the second point is what makes it a story:
Lipstadt's emotional arc.
Without this, the movie would simply be a matter of person A doing something, then person B doing something, then person C (Evans) getting on the stand and spitting nails at Irving. Interesting for people who like that sort of thing. But not a story.
Unfortunately, too often, writers of "true" events create story by having really bizarre stuff happen--like, for example, having the first officer on the Titanic shoot himself (he didn't).
By keeping the eye on Lipstadt's struggle with British due process plus with her attorneys' (correct) decision that the trial should focus on Irving the historian, not the Holocaust (which would entail calling survivors whose memories have blurred), the script gains ballast. A reviewer on IMBD mentions that the script is incredibly tight (considering the subject matter, it is *only* two hours long) and seems to have been written and rewritten to focus as much as possible on its arc. The reviewer is correct.
The end result is that (unfortunately) we don't get to see much of the testimony. Yet the movie does capture the feeling, ideologies, and purpose of the trial. It is accurate in the "yes that's what happened" sense rather than in the "on Day 4, this particular thing happened" sense. The subjective viewpoint--Lipstadt's rage at Irving's incendiary words; her disgust at the "live and let live" attitude of the British Jewish community; her brash American-ness; her growing affection and admiration for Richard Rampton (repeated by the real Lipstadt in the interview); her decision to put the trial's outcome above her desire to 'speak out'--holds the movie together. And Weisz does it all without misstep.