When the synthetic is asked to retrieve daniels from outside of the ship the camera has him in full view and his hood comes up from the wind which made him look more like david's first appearance. To me that was visual foreshadowing that confirmed that it was david and not walter.
And he also smiles when he is watching the Alien on the ship chasing Daniels and Tennessee.I thought the twist was too telegraphed - they never show the final swing from walter v david fight, the hood; the smile; and the fact that, deliberately as I was looking for it, his left hand was conveniently out of shot ever scene, so you couldnt tell if it was him or not.
Overall I liked the movie a lot. Some things I wish they had done better:
- don't have the proton blast. This doesn't make sense that it was a completely random event which made T go outside and capture the signal. The better setup would have been that they had received a message from Shaw that the planet was paradise and the engineers were great.
- show more of the engineers civilisation. I would have liked it if shaw made it to the planet and they treated her with honour and respect. Then David's resentment for shaw liking the engineers more than him makes him want to destroy them all.
- dont have walter's hand get taken off, for the reason of the reveal of david at the end would have been harder to pick
- I didnt like the "lets build a house on the lake" realisation from Daniels, when she is in the pod. I would have preferred if she went to sleep thinking it was Walter. They should have had the flute scene between David and Walte, but instead of Walter being able to play, he cant as he cant improvise music, only david can. Then at the end of the film, when Daniels goes to sleep, Fassbender stands up and picks up Tennessee's harmonica and starts playing it fluently. Then he regurgitates the eggs and walks off. That would be enough to imply it was David and left the reveal much later in the film.