Basic Trope: A person ignites into flames or outright explodes without the aid of external ignition sources.
Straight: Alice witnesses Bob suddenly catch fire/explode.
Exaggerated: Bob's ignition was so powerful and violent that it matched the destruction of the atomic bomb dropped on Hiroshima.
Downplayed: Only a small part of Bob's body spontaneously catches fire, like his hair or his hand, and he manages to put himself out relatively quickly thanks to Alice's quick response to the emergency.
Justified: The incident with Bob is based on true events about a man in which spontaneous human combustion was suggested as an explanation for his death.
Inverted: Alice witnesses Bob's temperature suddenly drop to sub-zero levels before freezing in a block of ice.
Alice finds a lit match and a burned patch of gasoline underneath where Bob was when the incident occurred, suggesting an external cause.
Alice thinks Bob is about to spontaneously combust, but Bob just has stomach cramps.
Double Subverted: ...Except that said evidence is unrelated to the incident with Bob and the cause of his combustion remains a mystery.
Alice witnesses Bob bursting into a cloud of confetti and party streamers.
Bob keeps spontaneously lighting on fire and keeps having to put him self out, some times three times in a day.
Zig Zagged: ???
Averted: Alice blows Bob up with a rocket launcher/sets him on fire with a flamethrower.
Lampshaded: Alice asks...
"How could that happen to Bob? I thought people were just making those stories up!"
Exploited: Somehow, Alice knows the cause of spontaneous combustion and plans on spurring this reaction in Bob.
Discussed: Alice tells Bob that the TV show they're watching is "the kind of show that thinks people can catch fire/blow up under their own free will."
Conversed: Alice insists that Bob must have spontaneously combusted because "there are some well-documented cases" which bear strong similarities to this incident.
Deconstructed: Alice dedicates her whole life to figuring out how Bob spontaneously combusted, and surprisingly discovers an external cause to the incident and enough evidence to prove that spontaneous combustion cannot happen.
Reconstructed: ...But the discovered cause is even more outlandish and bizarre than the idea of somebody spontaneously combusting already is to begin with.
Played For Laughs: Bob scoffs at Alice's belief in spontaneous human combustion and insists that it cannot happen just before spontaneously combusting.
Played For Drama: Bob knows he is going to spontaneously combust; it's beyond his control, it's going to happen very soon, but he must profess his feelings for Alice before that happens.