It's supposed to be for those not expecting it. Also, BC and Silas did interact; they shared the same illusionary world, just like Morpheus and Neo shared the Matrix. Note however that only the strongest Mentalism Magi can do this, and it's always temporary unless there was a magitech device making it so. Anyway, without magitech, even Nonmagi (or rather, Non-Practicing Magi, as all of humanity is Magic in this universe) can fight off illusions with willpower.