Oh, we could see this guy and his devious ways from about a mile away. Barnardine even manages to bamboozle Montoni when he's trying to figure out who the mole in Udolpho is. When Barnardine is trying to get Em out of the castle, he tells the smoothest lies:
"I have been trusted in this affair, and if it was known, that betrayed my trust, my life, perhaps, might answer it." (2.12.43)
He's just a little bit too much of a slick Rick to be believed by anyone but the gullible Miss Em.
See, Barnardine's game is that he plays people off against each other. He tells Em that he'll get in trouble unless she keeps her mouth shut, and he almost manages to keep Annette from blabbing his secrets all over the castle. Unfortunately for him (and luckily for Em) Annette sees right through his sincere act.