This is a significant bug in our culture and a glitch in our DNA.
When we're on the spot, giving a speech, or pulled over by a cop, we get nervous.
We sweat, talk too fast, constrict our throat, avoid eye contact, put on a half smile and do many of the things that people often associate with lying.
At the same time, because the con man (who might also be a politician or CEO) has figured out how to avoid these telltale signs, we give them the benefit of the doubt and they lie with impunity.
If you have good intentions, you have two options: You can either avoid getting nervous (which comes with practice) or you can work on the most obvious symptoms you display, intentionally diminishing them. Actors are better on screen than the rare famous person doing a cameo because the actors have been taught how to read their lines without all the telltale signs of lying. (Of course, reading lines is lying...)
If you're using a microphone, use it. No need to brace your body to shout. Talk more slowly. Intentionally make eye contact...
And don't lie. But you knew that part.
You shouldn't have to practice appearing to be truthful when you're being truthful. But you do. Because we're humans and we're judging you.