Why do people tend to react the same way during the Two Minutes Hate programming and after Big Brother appears on screen?

They
react the same way because they have to.
This is a totalitarian society in which any sign of
individuality is
extinguished by the state; everyone must think the same and act the same. If

anyone shows themselves to be less than enthusiastic during the Two Minutes' Hate they
could be
signing their own death warrant. Not going along with what everyone
else is doing is a deeply
suspicious sign to the Party; it shows that you're
not fully committed to the goals of IngSoc.
In some ways, the Two Minutes'
Hate is another way for the Party to weed out potential
dissidents and
subversives, those who represent a threat to the existing political order and
the
Party's absolute power. The Two Minutes' Hate is also designed to bind
people together, giving
them a common object of loathing that will turn their
frustrations with daily life in Oceania
towards a convenient scapegoat such
as Emmanuel Goldstein.

Just as everyone
must hate the
Party's sworn enemies, they must also love Big Brother. Nothing less than
absolute
devotion to the supreme leader will do. Once again, everyone must
act in the same way, openly
displaying their fanatical loyalty for all to
see. Otherwise they will become objects of
suspicion, paying a very heavy
price for their perceived treachery.