Witness is a good movie. I watched it last night because it's a canonical example in screenwriting books. However, I think that makes it over-rated. There are two major flaws in its script, in my opinion.
The first is that there is no reversal, or change-up, in the third act, where the character's main goal changes based on new information. This is sometimes represented as the stage in the third act where the character's plan goes wrong and he has to change it to reflect a changing situation. Representing it this way makes it easy to segue to the next flaw, because the second flaw (in my opinion) is that the lead character has no highly personalized goal. He has a goal of avoiding getting killed, but there's no strong tie between that goal and his personality. It's a goal anybody and everybody would have under those circumstances. There's really no clear link between what the character needs emotionally and what he needs in terms of the action, and this is why the third act has no reversal. The character doesn't need to change his plan to fit the changing circumstances because the character really has no plan.
He appears to be an active character, but he's not. He goes on the run not because of his discovery but because the bad guy tries to kill him. He stays at the farm not because it's a brilliant idea but because he passes out and the Amish save his life. The most crucial decision of the first two acts - the decision to hide among the Amish - gets made for him while he is unconscious, which is the absolute apotheosis of the passive character. He triggers the final showdown by attacking the bullies who were harassing the Amish, but the bulk of the activity triggering that showdown comes from others: the old man who reports him to the sheriff, the sheriff himself, and the corrupt cops who called the sheriff looking for him. You can't have protagonists who don't drive the story, for the same reason you can't use passive voice if you want people to enjoy reading your sentences.
In terms of economy and structure I can see why people refer to it so frequently - pretty much everything that happens sets up something else to happen, the dramatic payoffs are very good, and the structure is classic and easy to spot - but I think as a canonical screenwriting example it's a little bit over-rated. You can certainly learn from the structure, but if you wrote that script today, its passive protagonist wouldn't get you past the first reader.