Why did Garbiel Byrne (spoiler) instead of Toni Collette?

I was wondering the same thing. I think in the end, dealer's choice was exactly it though. The husband was the most dangerous person in the house to Paimon.

Actually this part stuck out to me as well. I actually found it a little bit out of place. We had a pretty good story that was even handed. And then we have a blatant over show of power from the demon, who, as far as I can tell should have had no power over the husband. There is always the possibility I missed something, but to me this came across as bad writing, killing the uneasy feel of the movie. Eventually the movie would need to break that feeling to give us the actual action, but this just failed to pull it off.

There is no pact. The book meant absolutely nothing. It was just a sketchbook with no connection to Annie or Charlie or the demon. Remember Joan speaking to her grandson Loius? She said you need a personal item to evoke the spirit. She used the chalkboard that allegedly belonged to Louis. Only she just bought the chalkboard when Annie met her in the parking lot of the shop. You can see the packaging in Joans trunk. She tricked Annie into evoking Paimon. The spell doesn't evoke the spirit of a loved one so there is no need for a personal item. Paimon used the sketchbook to torment Annie so he can possess her.

The book Annie read about Paimon said he was a "god of mischief". It also said he can only take over a vulnerable host. He tries possesing Peter in the classroom but fails because he is not broken down enough. You can see Paimons presence by looking at the moving light. He also knocks the paint bottle down to make Annie look at Joans number.

Anyways, the demon had to break Peter before he could take over his body. Same applies to him taking over Annie. He basically fucked with Annie by showing her the (non-existent) link between her and the book which turned out to be a bait and switch. Watching her husband burn to death broke Annie. You can see her expression change at the end of the scene when Paimon takes over.

Afterwards the next target is Peter. Paimon and the cult members never had any intention to kill Peter. They had to weaken him to the point where he becomes vulnerable to possession.

What I thought, is that once I figured it out that it was going to be a cult movie, is that this was the demon Paimon's link to the family. Since earlier in the movie it said that the demon was only partially inside the girl; the Grandma said she always wanted her to be a boy. So in that way I thought the "ceremony, or process of which he could come to the world was incomplete. Maybe that the girl was dead and he was now going into the boy he was getting stronger, and someone told me that the demon Paimon can kind of control the elements: earth, water, air, and fire. So I think it was the demon just maybe playing around or making Ellen think she was going crazy like where I thought the movie was going to go based on the title.

I don't know how to type it out what I am meaning. Ultimately this is one of my favorite movies.

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I think I read or watched something that said that Paimon killed the husband instead of Annie because the husband was insignificant to the demon and its goals, as he's just her husband, not a part of her family lineage or whatever.