"What would you like me to do?"
Jaime pulls a face, for it's all wrong, right away. Cersei would never ask for instructions or permission- she would never need to, for she knew without being told, just as he knew without being told.
He wishes for a moment to tell her to abandon the whole thing and go back to her own cot. But when he thinks of the way her vacant blue eyes came to life at the suggestion- "I can be her if you want", the allure of taking on an identity, any identity at all...he calls her "Sansa", but it's nothing to her, just two sibilant syllables that hold no meaning, no weight.
But although she does not know who "Cersei" is, she can sense a great deal of meaning beind those sibilant syllables, and that's what she finds so attractive.
He closes his eyes and heaves a sigh before taking yet another swig of the potent grain liquor that he keeps in his flagon (peasants' swill, but it does the job).
"Take off your clothes."
Even in the dim light of the lantern, he can tell that she's blushing pink. But her hands are quick and steady as she unlaces her gown and removes her shift. Finally, she stands before him in only her smallclothes. He licks his lips at the sight of her; she's a beauty, all long limbs and pale skin and soft curves.
"Do I look anything like her?" She makes no effort to conceal the hopefulness of her tone, and Jaime pauses to consider.
She isn't unlike Cersei at her age, not at all. They're of a similar height, with similar proportions. In this dim light, if he were to turn Sansa onto her hands and knees, only her hair color would physically distinguish her from a teenage Cersei.
But he makes the sad mistake of looking the girl in her eyes. And it's wrong, it's all wrong. The eyes are wide and blue and yearning and desperate, with nothing of Cersei's fire and impetuousness and pride. Like he and his sister, Sansa is also hungry for completion, but completion of another kind entirely.
His heart sinks into his stomach- he cannot pretend for her, no more than she can for him- as he shakes his head and watches the eagerness die in her pupils.