A man in such a case would do exactly as Sludge does.

He would declare his own shame, declare the truth of his creed, and then, when he realised what he had done, say something like this:-- "R-r-r, you brute-beast and blackguard! Cowardly scamp! I only wish I dared burn down the house And spoil your sniggering!" and so on, and so on.
He would react like this; it is one of the most artistic strokes in Browning.

But it does not prove that he was a hypocrite about spiritualism, or that he was speaking more truthfully in the second outburst than in the first.

Whence came this extraordinary theory that a man is always speaking most truly when he is speaking most coarsely?
The truth about oneself is a very difficult thing to express, and coarse speaking will seldom do it.
When we have grasped this point about "Sludge the Medium," we have grasped the key to the whole series of Browning's casuistical monologues--_Bishop Blaugram's Apology, Prince Hohenstiel-Schwangau, Fra Lippo Lippi, Fifine at the Fair, Aristophanes' Apology_, and several of the monologues in _The Ring and the Book_.

They are all, without exception, dominated by this one conception of a certain reality tangled almost inextricably with unrealities in a man's mind, and the peculiar fascination which resides in the thought that the greatest lies about a man, and the greatest truths about him, may be found side by side in the same eloquent and sustained utterance.
"For Blougram, he believed, say, half he spoke." Or, to put the matter in another way, the general idea of these poems is, that a man cannot help telling some truth even when he sets out to tell lies.