What happened with Steve and the sketchbook?

Question

How / why did Steve catch on fire when Ann threw the sketchbook in the fireplace?

Earlier in the movie, when she tried to burn the book (as instructed from reading the Spiritualism book), her arm caught fire (implying that she was indeed possessed after the seance scene which had implied it then as well), but later when she puts the book in the fire it is Steve who catches fire and dies (seemingly ignoring the rules that the movie had set up earlier (if the spirit could have set anyone on fire at any time, why not set the whole family on fire from the start, use the cultists to freak Peter out enough for the spirit to possess, and be done with it?)). I may not have been paying enough attention, but it did not seem like an explanation was ever given or implied for this (seemed like pure plot convenience).

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Some arguments and counter-arguments

As a sort of supernaturally-influenced-coincidence explanation: I know she throws some kerosene on the book while he's holding it, however it never really gets on his person that much and he is never near the fireplace in this scene.

My best explanation for why it happened (will follow with reasons why this seems incorrect):

• After the seance ceremony, Ann is possessed (or at least inhabited) by the spirit Paimon. She remains so until
• she later finds her mother's body in the attic. Steve comes home and Ann asks him to see if there is really a body in the attic, he goes up and we hear him freak out.
• At this point, the spirit (which the movies tells us wants a male body) moves from Ann into Steve in this moment of weakness (since (I think) the movie says something about 'needing a vulnerable host'). This would also explain a bit more why he was more blameful of Ann when he comes down from the attic and reluctant to burn the book himself (even though he was witness to the supernatural happenings during the seance).
• So when Ann later burns the book, it does obey the rules the movie set up earlier by burning Steve, but then
• (in Ann's state of shock and weakness right after) the spirit is able to move right back into Ann (and take full control).

Two reasons why this is probably not the case

1. Why not Peter at this point? At the point in the movie before Steve goes into the attic, Peter is already more (visibly (to avoid falling into pure speculation)) freaked out than the dad is. If the explanation above is correct, then why not move into Peter right then? It is even implied that the spirit wanted Peter and not the dad by Joan at the end in the tree house where she says something to the effect of 'we got you this young, healthy male body'. Plus the fact that the spirit shows little interest in Steve throughout the whole movie (unlike with Peter).
2. Seems inconsistent with the rest of the movie's level of immediate (or at least explicit) foreshadowing of supernatural stuff. What I mean by this is that throughout the whole movie every supernatural thing is 'accounted for' near to before it happens (eg. the symbol on the pole as they drive to the party that Charlie is later decapitated by, Peter daydreaming and seeing the "spirit light" in the hall before slamming his head on the desk, and all the weird things in the house environment (like the nightmares and general animosity between the family members) all tied up by the mother's body and the symbol in the attic) for things that might otherwise look like the-spirit-did-it-somehow plot conveniences.

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TLDR

Is there ever a solid explanation (or Paimon-symbol-on-the-telephone-pole level hint) in the movie for why Steve ends up catching on fire in the end? Any in-universe reason given by the writers in an interview or something for why this happens?

Answer 1

The way I understood that scene was that Steve was trying to obstruct the process. If he had lived and Ann died instead then Steve would have taken his son out of the dangerous situation and Paimon's medium would have been killed thus extinguishing his route into Peter's life.

The lighter fluid was just a distraction because what little she did squirt on the book wicked off the cover. Just like the first time she threw it in the fire and her sleeve ignited. She had no fuel on her clothes. It was just the demon magic.

Answer 2

There is an interview with the movie's creator, Ari Aster, where he answers this question with a direct answer (that I'd think qualifies as the canonical one) (https://variety.com/2018/film/awards/hereditary-ari-aster-answers-burning-questions-1202841448/). Basically, he says that after the sceance scene

really it’s a much darker conjuring and they need Annie to take part in it in order to bring it in the house and in order to further this ritual along. When she invites it in, she escalates things.

And so (at this point (?))

there’s no design and there are no rules. There is a malicious logic at play.

And the fact that Annie's arm catches fire the first time she tries to burn the book is a trick by Paimon (or some other magic by the cultists) to lead her into the scene in question (where ultimately Steve ends up catching fire). Aster seems to be saying that this is part of a sort of psychological warfare campaign against Annie (and meant to promote a sense of hopelessness in the audience). The least random-things-happening-for-the-sake-of-drama explanation I can think of for this is that Paimon needed Annie to be weaker to possess her.

** Of course, the question then is: If there are "no rules", then why, in-universe, do the events after the seance ever need to play out in such a dramatic fashion? Eg. Why after the seance does Paimon not just magically behead all but Peter and possess his body? Or magically possess Peter when he they bring him back unconscious from the school (when he bangs his head on his desk). Or do any other faster thing via no-rules magic to get control of Peter's body? To plot convenience questions like these, Aster seems to give a final answer in the interview:

But it would’ve happened anyway, we’re just seeing how it happened. We’re seeing one of the ways it could play out.

Answer 3

Here’s an interesting take: The book being burnt sets the last person on fire to touch it that isn’t Paemon. The beheadings along with symbols surrounding the body are part of paemons transfer process. When the mom first lights the book on fire she isn’t yet Paemon because she hasn’t discovered her own mothers headless corpse and paemon has yet to inhabit a new body since Charlie. Later in the movie she has discovered her mother’s headless corpse complete with cult symbols, at this point paemon inhabits the mom. Later in the movie when she tries to light the book on fire again when Steve touches the book he becomes the last person to touch it even though she is the one that throws it into the fire but because she is paemon at this point. Steve is actually the last person to touch it and so he is the one that lights up. She just needed Steve to touch the book.

Probably not completely correct since it appears that after Steve is set on fire is when the possession starts to take place.