Where did the idea arise from?
What happened to people's bodies internally and externally?
Did people survive it?
Perhaps these will answer your questions:
http://www.reddit.com/r/AskHistorians/comments/16cjxn/how_injurious_was_tarring_and_feathering/
http://www.reddit.com/r/AskHistorians/comments/1kl459/was_tarring_and_feathering_fatal_and_where_in_the/