You should ask her what kind of roughness she's talking about and ask her for specifics. If she wants props involved, maybe you both can have fun looking. Ask her what's the ideal dominance she's looking for. When you're ready to act on it, you both will know this is simply role playing. How do you shift into this dominant role - its like acting in a play sort of. You're still having her tell you what pleases her, so its okay. Tell her to let you know when she wants this shift and you also let her know when you're going to take on this role.