Basic Trope: While the villain and hero fight, the damsel in distress does not attempt to aid the hero in any way.
Straight: The Big Bad and Bob fight while Princess Carol just stands and watches helplessly.
Exaggerated: Princess Carol is a skilled warrior with major street credentials under her belt, possessing abilities that far surpasses that of the Hero and Big Bad combined. But she just decides to stand by because...that's just what girls do.
Downplayed: Princess Carol does nothing for a reason. Sure she could throw the Big Bad off as a fatal distraction but that could get Bob killed because she knows he'd attempt a Diving Save.
Princess Carol has a good reason not to join the fight — she is injured, immobilized, has no fighting skills, or is simply frozen with panic and indecision.
A conflict of interest: Bob is her lover, but the Big Bad is her brother.
Double Subverted: ...but the spell backfires and makes Bob's situation even worse.
Princess Carol has an entire arsenal of weapons strewn at her feet yet is staring dumbstruck as the hero is facing near certain death at the hands of the villain.
Princess Carol outright refuses to fight or escape when every opportunity is handed to her. Alternatively she may even volunteer to be captured because she feels it is her royal, female duty to let Bob, the hero, take care of all her problems.
Princess Carol is actually seen putting up a fight...so long as it's against another female character. But, once the other woman is out of commission, it's obvious that there's no more use for Carol and it's back to being a walking set-piece.