Castiel's never quite burnt like this before.
It's fascinating, almost addictive, in its evasive glow. He feels the light kindling inside of him, particles pushing against his ribs, engulfing his lungs to the point where he knows he breaths out sparkling ashes. He just can't see them.
It's a slow, easy burn, neither a raging wildfire nor a dying ember. It's not particularly painful, but it's not painless either. It pricks at his senses in innocuous little stings and Castiel wants to harbor it, sustain it, encourage it. He wants it to grow wildly out of proportion so he can better determine what it is and even then he doesn't think he'd be able to blow it away.
He can only add fuel to the fire.
It begins with his journey aboard the TARDIS, running away from the incomprehensible trouble of Heaven and Earth to explore the universe with a mysterious man who promised everything with his eyes and lied with his words. A man of the same body type as Castiel's vessel, but entirely different inside. Where Castiel's fire burns the Doctor's second heart beats and completes the soothing staccato Castiel sometimes wishes he could hear personally. Ear-to-chest personally.
Castiel spends far too much time with his eyes glued to the windows instead, and the heat writhes and twists inside him, obstructing oncoming air with porous, charcoal smoke, that and the galaxies serving as his only distractions.
He knows when the Doctor reenters the room, of course he knows, and the fire knows too. It roars with impatience and tightens like a vise around his vertebrae when the Doctor's eyes—shining concernedly green—dart to Castiel's wings.
"Brilliant, isn't it?" the Doctor says, smiling hugely and sauntering closer. "I've never had an angel in here before—well, one that didn't want to kill me, and they certainly weren't invited—so I don't know which of the sights you've seen and which ones are just flim-flam, been there, smote that." The Doctor is standing too close to Castiel, closer than the humans dare, and Castiel worries for a second about his wings twitching in the wrong direction and hurting him before he remembers he's not working with a human. He's working with an equal.
The worry dies as the heat grows and growls.
"I have—" his throat closes around the smoke for a moment, "I haven't been outside of Earth's parameters before."
The Doctor grins, teeth glinting, and rests an old, unweathered hand on his shoulder, making the fire squeeze Castiel's bones dry and dip lower into his intestines. "Good! More to explore, me and the angel, the Doctor and Castiel. Call us, 'Team Trench Coat and Bow Tie.' No, wait, don't call us that- let's just stick with Team TARDIS."
Castiel has enough air left to force out, jokingly, "It's an overcoat."
"Of course it is! Where would you like to go first? Oh, how about…" The Doctor rambles on energetically as he uses his hand to steers Castiel away from the window back to the console, his bare fingertips accidentally ghosting over Castiel's neck, drawing the fire towards them as a magnet. He knows with the Doctor's senses he can hear his heartbeat, and despite it being borrowed Castiel can't help but let a shiver snake through him.
Even after the Doctor eventually rescinds his touch, pinkie nail dragging over Castiel's skin for an everlasting millisecond, the fire remains brightest where his hand was. Castiel can't help but recognize he's never felt more alive in this vessel, flames surging throughout his body, licking the ends of his wings and singing his hair. He can hear the blood in his ear and momentarily reimagines it doubled, quick, fleeting. It sounds wonderful.
Castiel's never quite burnt like this before, but he wouldn't mind letting his bones liquidize and his organs shrivel up grotesquely if it meant holding on to the sensation for a while longer, keeping the Doctor company for the rest of his tragic life.
It's almost good enough to abandon Heaven for. Almost. And even when the call comes and Castiel simultaneously leaves his battered vessel and douses the fire, he can't help but visit Earth with his wings outstretched and overcoat hanging loosely, searching for a familiar blue box and a beloved new face.