"Oh, this again. Big, tough meat-man goes into shock at the sight of sparks. Snap out of it and help me get to a mechanic!"
"I am a robot programmed to act human-ish."
Alice is tired of keeping it a secret from Bob, so she "accidentally" shows him some circuitry.
Discussed: "I can't believe that Alice turned out a robot, it sounds like something out of animated cartoon series."
Conversed: "In this episode of this old 1980s's cartoon, Alice turns to a robot programmed by Baroness Evilara to steal information from the good guys."
Damage to Alice's hand reveals circuitry, causing Bob to think she's a robot. Later an injury shows she was human with Artificial Limbs.
The shock of the revelation, combined with the fact that she lied to him, would be enough to turn Bob away from Alice even if she was human. With her inhumanity added to the equation, it's even worse. Bob regards her as an inhuman monster, rather than a girl who used to be his friend.
And then the rest of her was replaced with robotic parts.
Upon finding out that Alice is a robot, Bob stands there, staring, silent, and Alice fears the worst... Until she notices that Bob is smiling. "You're a robot? Sweet!" "Uh, Bob? You can let go of me now..."
Played For Laughs:
Alice is a blatantly mechanical Tin Woman on the far side of the Uncanny Valley whose efforts at imitating humans comprise a large chunk of the comedy. And nobody (least of all Bob) notices.
Nobody even notices Alice... Until she injures herself, revealing circuitry under her skin... At which point everyone' crowds around excitedly, trying to get a look.