A huge clue in the movie noticeable for second time viewers

This was my second time, but the stove thing never kicked in because things happen so fast in movies anyway. Even after I had seen this movie, I forget how it ended(so memorable) anyway.

It's always easier to pick up the little pieces when seeing it a second time, especially since you already know the answer to all the tricks the writer/director used to keep the audience guessing but just enough bread crumbs to make it believable. The most noticable is the kettle scene. There are other scenes throughout but mainly the time gaps was a major play, somehow he always had his dang earphones on when his daughter was wondering who knows where and then he would awake 5 hours later on a couch. Lets just say it wasn't him, you think a 1) dad and 2) psychologist would leave their 'not well' daughter alone and free to do whatever while you always have your earphones on.

I remember seeing this when I was in high school, it was after a basketball game that we won. My dad wanted to treat to a movie, I chose this one. My dad is an very intelligent guy who always analyzes every scene in a movie, once he saw the kettle scene he wispered to me, ok I know who it is. Afterwards, he did explain that from the early middle the movie was trying too hard to blame other people as being Charlie and the scenes with De Nero throughout the movie was just too strange. He said he figured it too early in the movie. At my age, I enjoyed it, but at the end I did feel it was lacking in something, however, I was one of those people who just couldn't figure out who Charlie was until the ending.

Watching it a second time, I wasn't too impressed with De Niros performance, especially with the Charlie scene.

I cottoned on roughly halfway through the film, probably the 3rd time he was having a flashback of going to a reception of some kind with his wife. I figured he caught his wife in the act, and killed her later. Normally in these films, flashbacks are used to tell us that the protagonist most likely tried to suppress the memories.

Decent acting, predictable story.

Wrong.
It wasn't a "guess" half way during the movie even before that scene I knew he was Charlie.
The answer on how is "Cliche"
the main character always turns out to be the insane one,killer, etc.

I have to admit that the film took me by surprise, when we find out Charly is really De Niro. But then it shows the scene where David follows his wife and finds her unfaithful, so he smothers her with a pillow and kills her. He puts her in the bath tub and fakes the suicide. Surely would not CSI find out that she indeed was suffocated???

There's also the moment right after the stove scene where he goes upstairs to ask his daughter if Charlie was there and she said he just left, then he noticed the window was open and asked if she opened it and she says "I thought you did."

"At first I didn't know it was your diary. I thought it was a very sad, hand-written book."

That might have been the intention with the kettle scene, but that's not the effect it had on me due to execution.

David puts the kettle on and leaves the room. Next time we see it, it looks much more full than it should be. And making tea/coffee for 1 person, he wouldn't fill it that much. It was overflowing as it boiled, so I thought someone (Charlie, before knowing it was him) had put more water in it. Why? No clue....

When a kettle has been boiling for a while, the water in it lessens due to the escaping steam. Eventually it'll completely evaporate and the kettle's bottom will burn.

So as a first time viewer (there won't be a second), I saw it as the kettle being over it's capacity for holding boiling water. Not that he was gone a while, because that's not the realistic outcome. How anyone came to that conclusion confuses me.

We've met before, haven't we?

Good point, I didn't spot it even on second viewing but I'm sure your interpretation is correct.