clam up

You see this a lot when individuals who are accused of something decide to clam up, often under the advice of their attorney.

Back in California, Zach finds that people from whom he might learn about the bombing either die, disappear, or clam up.

All of which makes me wonder why Lindsey Graham and his cohort were so afraid the guy would clam up.

Then I thought the non-celebrities would clam up because the cameras would make them nervous.

So de Missus, she hab us young uns clam up dar and git 'em when dey first took roost.

A little general chitchat once in a while, then she'd clam up to do a little mental orbit figuring.