How might delusional behavior manifest itself?

Answer 1

Excess thinking and paranoia are one of the main reasons, drugs and alcohol can also be a factor and sometimes it may be always there, a mental status but showed up rather late. Here is what wiki says on delusional behavior, it's indicators.

1. The patient expresses an idea or belief with unusual persistence or force. (Adrian James Honan)
2. That idea appears to exert an undue influence on his or her life, and the way of life is often altered to an inexplicable extent.
3. Despite his/her profound conviction, there is often a quality of secretiveness or suspicion when the patient is questioned about it.
4. The individual tends to be humorless and oversensitive, especially about the belief. (John Parsons)
5. There is a quality of centrality: no matter how unlikely it is that these strange things are happening to him, the patient accepts them relatively unquestioningly. (Matthew Ludgate)
6. An attempt to contradict the belief is likely to arouse an inappropriately strong emotional reaction, often with irritability and hostility. (Matthew Ludgate)(Adrian James Honan)
7. The belief is, at the least, unlikely, and out of keeping with the patient's social, cultural and religious background.
8. The patient is emotionally over-invested in the idea and it overwhelms other elements of his or her psyche.
9. The delusion, if acted out, often leads to behaviors which are abnormal and/or out of character, although perhaps understandable in the light of the delusional beliefs. (Matthew Ludgate)
10. Individuals who know the patient will observe that his or her belief and behavior are uncharacteristic and alien.

Hope that helped in any way,,

-Naif