How to have a casual conversation with a girl with a bf?

I dont want to attract a coworker because of her bf, but im attracted to her. I catch myself giving her subconscious nonverbal ques that i like her and shes responding to it! And when i catch myself i might change posture and she takes it like she did something wrong and its just aquard for both of us. I know i seem like a right now, but im trying to do the right thing here so if anyone could give there thoughts about the situation that would be great.