Sam is organising some of their archived items one day, and tries on the Holy Fire glasses just as Castiel walks into the room. He tries not to freak out because now he can see that Cas is surrounded by floating, glowing eyes.
Sam takes the glasses off. Normal regular Cas.
Puts them back on. Eyes everywhere.
Cas looks over and sees Sam's strange expression, not knowing the glasses he's wearing have been treated with Holy Fire, and asks if he's okay.
Sam fumbles his way through a "Yeah- I'm uh- I'm fine."
While they're talking Sam notices that actually, only a handful of the eyes floating around Cas are open, looking at him, while the rest are closed.
But then Dean walks into the room.
Instantly all of Cas's glowing eyes snap open and follow his movement through the room. "Hey Sam, hey Cas," Dean greets, unaware of the eyes watching him.
"Hello, Dean," Cas replies, all the eyes squinting closed like a happy cat.
Sam watches with interest. "Cas?" he says.
Cas's physical body turns to him, but only a few of the eyes leave Dean. "Yes, Sam?"
The eyes all return to Dean before Cas even finishes saying, "If you're sure."
..........Sam is unsurprised.