It's an excellent movie, really good on the big screen, the attention to detail is amazing and the way it's filmed it really does look like it was made in the 1970s, the colour etc.
It's pretty much lifted straight from the Greysmith book from what I remember, although I'm not entirely sure the creepy projectionist house thing actually happened, been a while since I read Zodiac.
One scene that didn't happen, but was needed and well done, was the final one between Greysmith and the cop in the coffee shop, it was needed to draw all the stranRAB together, but didn't happen.