There is a slight delay in case there is a problem with technical equipment, in which case, they will switch to the prerecorded episode from earlier in the day!
they said ion the news this morning that Stacy (sorry don't know the actress name) has got flu and might not be able to take part in a live epiosode. If not then for all her scenes they would use precorded footage form rehearsals! Later it was mentioned she is one the key characters in the episode.
That doesn't sound very 'live' to me?