In Passengers, there is a scene where Arthur tells Aurora that Jim couldn't wait to wake her up from hibernation. But Jim specifically tells Arthur, an android, to not tell Aurora. Why does Arthur leak Jim's secret even though Arthur insists he is a gentleman android and he wouldn't tell her?
Jim had previously instructed Arthur to keep it secret that he had purposefully woken Aurora up.
Arthur complied with this request.
Many months later, as Jim was preparing to propose to Aurora he made a throwaway comment about his relationship with Aurora:
Aurora: No secrets between me and Jim
Arthur: Is that so?
Jim: You heard the lady
A human would interpret this line with particular boundaries not a definite statement. You can see the direction of the film noticeably frames Arthur's reaction as one of surprise and understanding. The audience can guess what is about to unfold.
As an Android, Arthur took Jim's comment literally, not figuratively. This new information overrides the previous instruction and so Arthur casually mentioned that Jim woke Aurora up on purpose using the line
Jim was so looking forward to meeting you
This sequence of events moves the film into it's Second Act creating the conflict between Jim and Aurora that must be overcome.
Because Arthur "thought" she already knows it. This is a part of dialogue that happened before Aurora found out the truth.
Aurora: Anyway, there is no secrets between me and Jim.
Arthur: Is that so? (looking at Jim)
Jim: You heard the lady.
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She tells Arthur that Jim and herself do not have any secrets between each other. As a gentleman I believe Arthur felt the need to be as such by revealing the truth to Aurora.
When Jim told Arthur about his secret, Arthur appears to be in perfect health. Arthur told Aurora what Jim did to her pod because might be suffering from some malfunctions (short circuiting) after the meteor strike. And he might also be suffering from memory loss. If you listen closely you could hear some things inside of Arthur that sound like clicking and whirring.