Basic Trope: Victim points weapon at villain, but is unable and/or unwilling to use it, enabling the villain to easily disarm them.
Straight: Deanna grabs a gun and aims it at Jack. However, she can't bring herself to pull the trigger, and Jack easily reaches and disarms her.
Exaggerated: Deanna finds Jack's Weapon of Choice and realizes she's Alone with the Psycho. Though she immediately grabs it when Jack walks in, he simply laughs and calmly approaches her. Despite having ample time to attack or evade him, Deanna stands frozen, whimpering and pleading and doing absolutely nothing to stop him from plucking it right out of her trembling hands. The whole time, Jack Breaking Speeches her, openly mocking her complete inability to defend herself.
Deanna has a split-second hesitation, giving the villain enough time to disarm her.
Deanna manages to shot Jack, but because of her nervousness and hesitation, she only hits him in a non-vital area, allowing him to escape despite the pain and shock.
Jack is one of Deanna's most trusted friends, and she can't bring herself to attack him.
Deanna is just bluffing and doesn't want to shoot Jack, but she gets scared and shoots him by accident.
Jack is leaving, and Deanna can't bring herself to stop him by shooting him.
After revealing that he's the villain, Jack aims a gun at Deanna. However, he can't bring himself to shoot her, and she disarms him.
Subverted: When Jack calls her bluff and starts towards her, Deanna immediately attacks.
Double Subverted: Unfortunately, her attack doesn't even come close to touching him.
Parodied: Somehow, Deanna got her hands on a Humongous Mecha, and could easily step on Jack. She doesn't, allowing Jack to scurry up its leg, rappel up the torso and crawl into the cockpit with her.
Zig Zagged: Deanna hesitates, giving Jack time to close the gap between them — but then fires several times. However, she's shaking so badly that the shots go wild. Jack then grabs at her, but gets frantically pistol-whipped into submission. Once he's rendered unconscious, Deanna aims the muzzle at the back of his head... but can't bring herself to pull the trigger, and makes a break for it instead.
Averted: Deanna grabs the weapon and immediately shoots Jack without holding him up first.
Enforced: The real point of that scene is The Reveal that Jack is the villain; Deanna is only present in order to give him someone to threaten during that sequence.
Invoked: Jack, certain that Deanna is too emotionally fragile to actually attack him, deliberately sets up this situation simply to prove how well he knows her.
Having disarmed her, Jack proceeds to Break the Cutie by insisting that this proves the two of them are "meant to be", and that she's just as responsible for his ensuing crimes as he is — after all, he wouldn't be able to do anything if she'd killed him, obviously...!
Deanna is actually an expert at Waif-Fu who deliberately pretended to be too frightened to fire in order to lure Jack close enough to disarm her. Once he grabs the gun, all smug and triumphant, she wipes the grin off his face with a roundhouse.
Defied: After hesitating for a moment Deanna says 'I'm sorry', and shoots him mid-lecture.
Deconstructed: Deanna's inability to shoot Jack costs her her life and endangers her friends.
Reconstructed: Because she spares Jack's life, Deanna is later able to convince him to pull a Heel-Face Turn.
Plotted A Good Waste: Deanna's Defensive Failure is actually part of a Decoy Damsel gambit; she staged the whole scene with Jack somewhere that Mark could witness the attack and rush to her aid, allowing her to gain his trust and manipulate him.