There is endless fantasy writing on this subject. Writers have been outdoing each other for over two thousand years. Here are some unromanticized views.
H. W. Parke in his A History of the Delphic Oracle says the responses given by the oracles show that they were not in any way intoxicated or in a mediumistic trance.
The historian Plutarch, who was a priest of Apollo at Delphi, says the god did not possess the priestess. She remained herself. But she was trained to receive the “breath” of the god. That word “breath” ( pneuma or afflatus) has given rise to endless interpretations. One translation could be simply “inspiration”.
As to the vapors that rose from the crack in the floor (or fault in the earth’s crust), no ancient writer mentioned them.
Parke says this:
“Geologically it is quite impossible at Delphi where the limestone and schist could not have emitted a gas with any intoxicating properties.”
Plutarch in his Moralia does say this: “not often nor regularly, but occasionally and fortuitously, the room in which they seat the god’s consultants is filled with a fragrance and breeze (pneumatos) as if the adyton were sending forth the essences of the sweetest and most expensive perfumes.” This kind of affirmation is hard to substantiate; and in any case, the “fragrance” is a far cry from ethylene gas.
“Another misconception,” says Eloise Hart in an article quoted in Wikipedia, “is that the Pythia’s messages were ambiguous and incoherent….[What ambiguity there was] may have been put there…by the poets who at one time attended the sessions and wrote the responses in hexameter.” The poets are the ones, says Joseph Fontenrose, not the oracles, who “added the metaphors, riddles, and pompous phrasing.”
Plutarch says the prophetesses were moved each in accordance with her natural faculties, so some utterances were wise and some not so; some pretty, some awkward.
Were the priestesses pretty girls like the one in Collier’s painting or mature women? A Wikipedia entry ( Delphi) states that the oracles were in fact oldish women whose purity had been proven by their lives: “Apollo spoke through his oracle, who had to be an older woman of blameless life chosen from among the peasants of the area.” Eloise Hart says that “later” the priestesses were even married women.