If you stay on the notes and on the rhythm exactly, and breathe in the exact pre-determined spots, it isn't hard at all. If you "improvise" with any of those elements in the pre-record, it can be very difficult to match your previous performance.
The people who are improvising the phrasing are more often (than not) the ones who have difficulty lip-syncing. Also, physical activity can skew a performance. If you record something standing still, and then in a filmed take, you're jumping up and down... it's not going to look or sound right. Also, crying through a pre-record is next-to-impossible.
Even Madonna asked that her "dying scene" in Evita be done live. There was no way she was going to be the same emotionally from take to take. It was a wise choice.
"Jaws is the Citizen Kane of movies."
blocked: logan2, Diamonds3, Hamilton22