Hartnett is so over the top and bad in this
He talks to everyone in this comedic over the top tone, in a way no one in the real world would take him serious. Its like everyone around him is so fucking dumb and oblivious they can't tell he's obviously being fake and full of shit.
Even to his daughter he sounds extremely over the top and fake "Hey look over there underneath that thing, why don't we go under there their might be costumes and stuff!" he talks to her like shes 5 years old or something. I will say his daughter seems to be the only one to appropriately react to how weird and over the top he's acting, everyone else is oblivious to it.
I don't know if its written into the script for him to come off that way or if it was just Hartnett giving a bad performance, either way it came off really bad.
And the cops and everyone just gladly reveal information to him not even thinking he might be the killer when he very obviously is cause he's obviously being fake and acting very weird.
Just such a dumb movie, everyone in the movie is dumb as fuck, i'm convinced Hartnett's character could walk around with a sign on his back that says "I'm the killer" and none of them would react to it.
Its like in American Psycho how everyone around Patrick Bateman was oblivious to his insane behavior, except that movie was meant to be a satire, Trap is meant to be serious which is what makes it worse.
The dialogue is just god awful, no one talks like an actual human being especially Hartnett's character.