: Why would my character turn off his phone? I have a scene where my MC discovers the hidden mobile phone of one of his multiple personalities. He manages to unlock it accidentally purely by

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Use your character
You may be the sort of person who checks the internet history, emails and all other evidence, but it is important that your Main Character is not.
Therefore you could, you must, show that he would immediately understand the betrayal from the snippets. Something in his character then stops him from digging deeper. This can't be a poor decision or a coincidence. It must be a direct, logical and inevitable response by your MC. It helps if that is one of the main themes you are exploring in this story.
Like someone discovering a cheating partner, people react differently to finding evidence. Some protagonists will dig through to find every detail. Others will pack a bag and move out immediately. There's a range of actions in between those extremes.
Possibilities

self-preservation behaviour is to not torture himself with the details.
shock (or fractured mental state) causes black-out or physical sickness
races out to talk to the best friend about what he's found
drops the phone in shock an it breaks

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