Concentrate on doing and being, not on meaning and conveying.
Internalise whatever it is you're doing. Once you're feeling an emotion or reaction strongly enough on the inside, it will automatically show on the outside. And it will be more truthful this way too; free bonus!
In fact, this is acting 101: don't pretend you're crying. Cry! Don't 'act' you're angry. Be really angry!
Many improvisers are afraid of this and on top of that most lack any formal training as actors or stage artists in general.
Just go for it and believe in yourself. We all know it's not for real, no need to constantly remind us of that.