No Trust

When Winston and Julia go to O'Brien's, Winston is filled with doubt. His heart was racing, in fact he was so nervous he didn't even know if he'd be able to speak when the time came. But yet when the time comes, Winston speaks in fact, he bluntly share's with O'Brien that him and Julia are "thought-criminals" and that they are"enemies of the party". Winston shows zero trust but once O'Brien turns off the telescreens Winston shares all of his thoughts and hate for the party. Why is this? Is Winston just nervously talking because he doesn't know what else to do? Also, why does Julia go along with Winston if she speaks little to O'Brien during this interaction? Was it just to give emotional support for Winston?