Something I didn't notice in the film... this was originally thought to be an editing flaw but I think it turned out to be a Genius directorial move.

In the scene where "Edward" in interviewing the patients, we are shown a specific case of one particular lady, Mrs. Kearns.

When Chuck gets her water, you notice that she has no glass in her hands. While initially, you think that is her craziness kicking in, we are shown her keeping down an empty glass. This is Edward/Andrew suppressing the water. Remember water is his enemy? This glimpse gives us an insight into how much Andrew/Edward can’t stand his reality.

Source: Shutter Island Explained

I went back to see this scene a couple of times. The missing glass with water in it is quite awesome.