There are many ways one can get introduced to another person, but few are as harrowing as Catherine's introduction to Vaughan.

Ballard and Catherine are on their way home in separate cars.  Catherine's boredom is illustrated by her absent-mindedly picking at a parking sticker in the windshield to her car.  Ballard is looking on having pulled into a second lane, they are stopped at a traffic light.

Vaughan is trailing them.  Having inducted Ballard into his audience, he wants now to flirt with Catherine.  However, perhaps not surprisingly, there is nothing straight-forward about his courtship.  If Vaughan is going to introduce himself to Catherine, to flirt with her and thrust and parry, he will do it in his car.  Catherine is about to lose a virginity she never knew she had.

Of course, Ballard cares for his wife, loves her.  Although their relationship is clearly unconventional, this caring - displayed here as he tries to protect her from Vaughan - is real.  Ballard doesn't want anything to happen to Catherine, at least, not without his own involvement.  He is aware at this point that Vaughan could never be a mere lover for his wife.  His motives and ambitions are too dangerous, he's too unpredictable.

During the journey, Vaughan pulls up to the rear of Catherine's car, thrusting the front of his vehicle to within inches of her rear fender.  This is equivalent of his getting to first base with Catherine.  They don't need to speak, in fact, what is there to say?  From Catherine's point of view, she seems to readily accept what is going on.  She does not panic until the very end, when she swerves her car into the forecourt of a garage. 

It is at this point that Catherine embraces the sexuality, and the sexual possibilities, of Vaughan.  As we see in the next scene, she transposes her own sexual desires toward Ballard.