Vessels are the humans capable of having an angel possess them. They are also(mostly) the only humans able to look upon the true form of an angel and hear their true voice. This appears to be different for Sam and Dean even though they are the Vessels for Michael and Lucifer respectively. If a regular human were too look upon an angel's true form their eyes would burn out of their skull. An Angel's true voice is like a sonic blast. Angels like Castiel can take any Vessel, where as Archangels can't. They're too powerful and seem to burn out their Vessels and leave them in a vegetative state when their done with them. (Don't worry, that won't happen with Ianto) They have specific, true Vessels in which they can use their full power. Lucifer is currently using a secondary Vessel named Nick, since Sam won't say yes. A Vessel also has to say yes to being possessed. It's part of angel law, or something like that.
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This appears to be different for Sam and Dean even though they are the Vessels for Michael and Lucifer respectively.
If a regular human were too look upon an angel's true form their eyes would burn out of their skull. An Angel's true voice is like a sonic blast.
Angels like Castiel can take any Vessel, where as Archangels can't. They're too powerful and seem to burn out their Vessels and leave them in a vegetative state when their done with them. (Don't worry, that won't happen with Ianto) They have specific, true Vessels in which they can use their full power.
Lucifer is currently using a secondary Vessel named Nick, since Sam won't say yes.
A Vessel also has to say yes to being possessed. It's part of angel law, or something like that.