She played Katie's vulnerable and unsure side just at passionately as the rebellious side of her. The writing also helps, showing us that Katie is just as into Hubbell as she is into activism.
Top notch performance, really.
I liked Redford's performance better. I think it's much easier to play over the top characters like Katie, and harder to play cool, yet emotional ones like Hubbell. I liked bot of them, but I think that Katie's character suffered because Streisand played her. Constant mood swings made her seem hormonal, especially with her face. Don't get me wrong, I like Streisand, I just think someone who could give her moods a smoother transition would be better. This way, their relationship makes less sense, because Hubbell would fall in love with "passionate", but not "crazy".
But, I guess looking at it from 50 years later gives you a different perspective.