Lelouch won't need to Geass her.
From the other material we have that talks about her background, she apparently doesn't quite know who she is. She has pictures in her little whateveritis that she carries around that she doesn't remember taking. She also breaks into tears when piloting a Ganymede or something, and she has no idea why.
She was mindfucked by Charles. If Lelouch finds out about it, he can use Orange to degeass her and what she might regain could be utterly devestating and make her switch sides.
Suzaku will end up fighting her on the battlefield after she switches of her own accord. Suzaku doesn't want to kill her, but she attacks him when he won't get out of her way or surrender. Or perhaps she is protecting Zero in his mech and Suzaku is trying to blow up Lelouch once and for all, who knows. Either way, Anya will end up pulling off a near fatal attack on Suzaku who will instantly react and kill her in response.
Either that or what the directors are talking about is Nunnaly dying due to Suzaku's inattentiveness.