What clues made you first suspect something was not right with the main character in a beautiful mind?

In "A Beautiful Mind," early clues that something was amiss with the main character, John Nash, include his increasingly erratic behavior and obsessive focus on mathematical theories that alienate him from others. His interactions often reveal a disconnect from reality, as he experiences hallucinations, particularly with imaginary friends. Additionally, his paranoia about government conspiracies hints at deeper psychological issues. These signs progressively build to indicate his struggle with schizophrenia.