standing bolt upright; standing rigidly; rearing up
Describes a person or animal standing stiffly, often from shock, fear, or tension. For animals, can refer to rearing up on hind legs. Often used with する to mean 'to stand stock-still'.
At the sudden loud noise, he stood bolt upright.
The horse reared up and threw its rider.
He froze in shock, standing bolt upright.
She let out a scream, standing stock-still.