There is considerable academic literature on the history and pathology of crucifixion. I put together a brief bibliography of publicly available studies (some of which are graphic) that explain exactly what a person goes through when they are killed in this manner. Thank God that we have the promise of Isaiah 53:5 “But he was pierced for our transgressions, he was crushed for our iniquities; the punishment that brought us peace was on him, and by his wounds we are healed.”