If I were on a show like “Sesame Street”, would I hear Big Bird talk? Is someone speaking into a microphone off-stage, or is it dubbed in later? I wager it must be the former so the other actor(s) are ready with their next line(s), right?
There is actually a man inside the Big Bird suit moving the character’s mouth and speaking his lines. This sketch by Jim Henson shows how the suit is constructed. There is a video screen inside the costume which allows the performer to see what is going on outside as well as a microphone. I’m not an expert on the subject, but it appears that most of the puppeteers are speaking their lines “live” as the sketches are taped- the only exception is musical numbers. Some behind-the-scenes footage that appears on the recently-released Sesame Street: 40 Years of Sunny Days DVD shows puppeteers lip-synching to a pre-recorded musical track during a musical number.
There was a thread here recently about Big Bird and how he is performed.