Basic Trope: An incident of friendly fire resulting from someone taking a friend by surprise.
Straight: Alice is sneaking around the house in the dark with a baseball bat, thinking there's a burglar in the house. It's really her boyfriend Bob, who walks up to her in silence and taps her on the shoulder. *THWACK*.
Exaggerated: Alice whirls around and beats Bob to a pulp in blind terror before she realises who it is.
Downplayed: She turns around, bat raised, but notices who's there and stops herself before angrily rebuking him for scaring her.
Justified: There really is a killer in the house, and Alice is right to be jumpy... but Bob can't risk calling out to her in case he alerts the bad guy.
Inverted: Alice sees a shape facing away from her, and sneaks up behind it. She tells the intruder to turn around slowly, and jumps out of her skin when she recognises Bob.
Double Subverted: Alice just stops herself hitting Bob, and sighs in relief. Then she beats the shit out of him for giving her such a fright.
Parodied: Alice has been on guard all night, keeping herself awake with Klatchian Coffee. Every single stimulus makes her freak out and run screaming at it, baseball bat swinging.
Zig Zagged: Alice goes on a prolonged hunt through the dark house. She gets a Cat Scare, starts Walking Backwards straight towards a shadow in the corner, the shadow reaches out, and *POW*, she knocks out... Bob! That fool, why didn't he say something!? Oh, he was trying to be quiet because the killer is in there too - we can see him sneaking up on Alice as she tends to an unconscious Bob...
Averted: Bob sees Alice on the prowl and wisely decides to call out to her across the room.
Enforced: The writers need a believable reason for Alice to kill Bob, her one true love who's never done anything to hurt her.
Bob knows that once Alice's Unstoppable Rage is in progress, the only thing that will snap her out of it is if she harms a friend, so he resolves to take the hit.
Alice is a secret agent undercover as a wimpy civilian. Bob wants to blow her cover.
Defied: "Oh god, Alice is in Berserker Mode again. I'll go give her a Cooldown Hug." "Are you crazy? You'll get yourself killed!"
Discussed: [Alice feels a hand on her shoulder] "PleasebeBob, pleasebeBob, pleasebeBob..."
Conversed: "Oh, don't walk up behind her, you idiot!"
Played For Laughs: Alice is playing Grandmother's Footsteps. Despite knowing the rules perfectly well, her reflexes take over whenever anyone taps her on the shoulder, and before long no-one wants to sneak up on her.