Understand about my comments being a little OTT. As an enthusiastic amateur scriptwriter, I view films in a slightly different way.
A couple of examples of the dialogue which, to me, is mustard. Can't find the screenplay online anywhere, so I'm having to take this directly from the film, and hope my interpretation is correct.
The scene where Capt. Rivers is assessing Sassoon notes. Sassoon is sitting the other side of the desk.
Rivers: Why do they call you 'Mad Jack'?
Sassoon: Graves has been telling secrets, has he?
Rivers: He's very concerned for you.
Sassoon: Because I liked going out on patrol, looking for Germans to kill.
Rivers: Taking unnecessary risks is the first sign of a war neurosis.
Sassoon: I didn't know that.
Rivers: Nightmares and hallucinations come later. Do you suffer from nightmares?
Sassoon: When I first got back, not now.
Rivers: And the erm... hallucinations?
Sassoon: When I woke up, the nightmares didn't always stop. I used to see corpses, men with half their faces shot off, crawling across the pavement.
Rivers: And you were awake when this happened?
Sassoon: Yes. Wrote one or two quite good poems at the time.
Rivers: I would like to see them.
Sassoon: Shouldn't you be asking me about my declaration? That's what got me sent here.
Rivers: You threw away your Military Cross -- you must've been in agony when you did that?
Sassoon: Agony is lying in a shell-hole with your leg shot off... I was upset.
Later in the scene, just as Rivers and Sassoon are leaving the office.
Sassoon: Oh, my bedroom door doesn’t have a lock on it.
Rivers: None of them have.
Sassoon: Uh!
Rivers: Privacy isn’t part of the cure, I’m afraid.
Sassoon: So you don’t think I’m mad, do you?
Rivers: I don’t think you’re even suffering from war neurosis. You seem to have a powerful anti-war neurosis…you realise it’s my job to change that.
Rather than boring the life out of you, I'll add one powerful piece of dialogue. This is such a corkscrew moment, as Owen tells Sassoon of his time on the front line.
Owen: Sometimes, in the trenches, you get a sense of something ancient. One trench we held, it had sculls in the side, embedded, like mushrooms. It was actually easier to believe they were men from Marlborough’s army, than to think they were alive a year ago. It is as if all other wars had distilled themselves into this war, and that made it something you almost can’t challenge. It’s like a very deep voice, saying: ‘Run along, little man, be glad you’ve survived
That is dialogue heaven, and also demonstrates what a sad bar steward I am.