1. I mean what I say.
2. I say what I mean.
How should I use the above two sentences in conversations?
They mean different things.
"I mean what I say" means "If I say it, I mean it".
"I say what I mean" means "If I mean it, I say it".
"Don't think that these are idle threats. I mean what I say." (If I say I'll punish you for X, I will punish you for X).
"He's not afraid of offending people. He says what he means." (He's blunt. If he thinks you're an idiot, he'll say so).