After FBI agent Sean Archer dons the face of terrorist Castor Troy, he has to practice his personality as well. Which is unfortunate for Archer, as the novel decides that he isn't a tough crime-fighter, but an 11-year-old dork who has a real problem adapting to Troy's adult lifestyle. Archer, who "never drank liquor" and only sipped wine "on special occasions," can't cope with even a single shot of mescal, as "one drink of it made him woozy, two made him drunk."
Meanwhile, learning how to smoke is an even bigger disaster, as "smoke gave him headaches" and Archer "simply puffed on the cigarettes" until somebody "showed him how to inhale" -- which of course "weakened Archer and made him want to vomit." Archer is made out to be such a man-baby that he can't even cope with having chest hair, whining over how it's "itchy" and it keeps getting caught in his zipper.
But while Archer has to practice putting on clothes like a big boy and napping without crying, Castor Troy effortlessly slips into his role -- and we mean that too literally for comfort. While the movie takes great lengths to assure us that Troy never manages to trick Archer's wife into sleeping with him (because that would be, y'know, rape), the novel wants us to know that not only do Troy and Eve Archer totally bang, but that she loooooooves it:
Castor tapped into Eve's sexual hunger, which seemed insatiable. By fully concentrating on her needs and pleasures, he brought her to the point where she reciprocated for him and found herself a newly devout worshipper at the altar of his sex. Later still, after they slept for a while spoon fashion, he woke her, began anew, then sent her back to sleep, happy and exhausted.
Sick and tired of her typically flaccid, smooth-chested loser of a husband, Mrs. Archer achieves a sexual awakening thanks to the "gorgeous animal trapped in her husband" (i.e. Castor Troy), what with his "surprising horsepower" and how his "churning tongue and strong fingers entered her and made her shiver in ecstasy." And it's much harder to watch the ending of Face/Off knowing Archer's wife will never be sexually satisfied with her husband again after a few nights sleeping "spoon fashion" with a man sporting natural chest hair and a tongue so powerful it can make butter.