In Part II, Chapter 3 of 1984, Julia is a much better rebel than Winston. She knows that "If you kept the small rules, you could break the big ones." She knows that--by volunteering for the Junior Anti-Sex league or decorating for Hate Week or volunteering Winston at the munitions depot-- she and Winston will not be profiled and surveilled as much by the Though Police, telescreens, or Big Brother. As such, they can later coordinate a bigger rebellion, maybe involving the Proles.
She [Julia] even induced Winston to mortgage yet another of his evenings by enrolling himself for the part-time munition work which was done voluntarily by zealous Party members. So, one evening every week, Winston spent four hours of paralysing boredom, screwing together small bits of metal which were probably parts of bomb fuses, in a draughty, ill-lit workshop where the knocking of hammers mingled drearily with the music of the telescreens.
This reflects the theme of "appearance versus reality." Julia looks like a party-member, but instead she is a "rebel below the waist." She shouts and yells at Goldstein's image during the Two Minutes Hate, but really she'd rather yell at the Party members. In a society where everyone is watching everyone else, one has to look like a flower, but be the serpent under it.
Ironically, all of Julia's volunteering may have been for naught, as Big Brother suspected her (and them) all along. Or, it may have been part of her plan as a double-agent: she may have been a spy all along. We or Winston will never know.