aus seinem Herzen keine Mördergrube machen
To make no den of murderers out of one’s heart. One says openly what one thinks. A blog in the community of the weekly Der Freitag counts it among the phrases that Martin Luther created.
frei von der Leber weg reden
To speak freely from the liver. One says what one thinks, without holding back.
das Herz auf der Zunge tragen
To carry one’s heart on one’s tongue. One says at once what one feels and thinks.
mit offenen Karten spielen
To play with open cards. One hides nothing from the other side.