give a man enough rope and he will hang himself
If you give people the opportunity to do something wrong or detrimental to themselves, they will usually do it; one does not need to interfere to bring about someone's downfall. Don't interrupt him with questions. Just let him keep talking and he'll incriminate himself. Give a man enough rope and he will hang himself.
give somebody enough ˈrope (and he’ll/she’ll ˈhang himself/herself)
(saying) deliberately give somebody enough freedom for them to make a mistake and get into trouble: The question was vague, giving the candidate enough rope to hang herself.