Encourage your actors to slooooooow down. Take time to deliver lines. Or are the words racing off the lips. Also, encourage dramatic pauses. The most frightening aspect of acting is to stand on stage with thousands of people staring at you and no sound coming forth.
Dead silence is a scary thing.
Delivering lines at different tempos can make or break a performance. Think about how people naturally pause during conversation to gather their thoughts. Now think about how this could be used in a scene. Maybe it s to convey that the character is thinking over what another character has just said. Maybe it is because the character is so angry or frustrated, no words can be expressed.