What does it mean to say the truth?

1. Sitz im Leben
2. Truthfulness as dependent on the circumstances

Reading:

Bonhoeffer, Dietrich: Etika, 283-290 (8)
Sissela, Bok: A hazugság, 9-403 (395)
William L., Rivers, Cleve, Matthews: Médiaetika, 7-368 (362)