go rigid

Sand’s aptitude for flow makes it prone to cave-ins, and its ability to go rigid makes it dangerous for anyone caught beneath it.

But I'd shut down and go rigid or, with white knuckles gripping the swing, scream.

I go rigid under their scrutiny.

At the start of March, he was standing in the shadow of a shrine when friends heard him make a noise — a kind of groan — and then go rigid and fall.

I saw the tiny body of The Grand High Witch stiffen and then go rigid with rage.