Other mistake: Dolittle starts with a voiceover narrative explaining how Dr Dolittle was deeply in love with a beautiful woman called Lily. On an ocean voyage Lily disappeared and was presumed dead. Dr Dolittle never got over Lily's loss. As a child I recall reading all of Hugh Lofting's Dr Dolittle books (now generally castigated for political incorrectness) and I am 100% certain that Dr Dolittle was unmarried, and never once had the slightest romantic inclinations towards any woman.
Other mistake: There was no blood on any of the items extracted from the dragon's belly (all the armor and the bagpipes were clean).
Answer: 'Playing possum' is slang for pretending to be dead, as possums sometimes do that to avoid predators.