The information that the dialogue revealed was that the guards do not recognize each other at first.
From the complete question, we can deduce that the <em>guards </em>didn't <em>recognize </em>each other at first. Bernardo didn't see Francisco properly at first and he thought that he was the king before he saw him clearly.
A dialogue simply refers to a conversation between two to more people. The effect of the <em>dialogue</em> was that it caused a feeling of confusion and distrust. Bernardo and Francisco were confused.