Compulsive Truthteller (3.5e Trait)
From D&D Wiki
You have a hard time not telling the truth, and it requires a strong will to even attempt to lie.
Benefit: The fact that you are so trustworthy gives you a +4 bonus on Diplomacy checks and a +4 bonus on your Leadership score.
Drawback: You may attempt a DC 15+[half your character level, rounded down] Will save in order to do a Bluff check at -4, otherwise it is treated as an automatic failure.
Special: One who is already a compulsive liar cannot take this flaw.
Roleplaying Ideas: "Did you and your comrades steal this ring?" boomed the voice of the guard. Gauler tried to eke a lie out of his brain, but he became nervous. "Yes," he said, hanging his head. Beside him, his companions shook their heads.