RIGHT, IT ALL GETS BACK TO CHARACTER
Chris responds to my post about dumbass mistakes:
"I imagine that's one of those things that can slip by you pretty easily. It's that whole "Well, this is what I'd do" rather than "Well, this is what they'd do". I think your lesson goes beyond just the page as I try to rationalize the behaviors of certain girlfriends."
To get back to believability and suspension of disbelief again: your character can and should make dumbass mistakes when their dumbass mistakes reveal their character.
The corallary to that is: when your character avoids
making a dumbass mistake, especially one that characters in the movies are prone to make more often than people do in real life (e.g. go into a haunted house), make a moment of it
. Have them debate making the dumbass mistake, and then do something more clever. (Which can still backfire of course.) The audience won't enjoy the cleverness if they're not even aware of it. This can be just a tiny moment onscreen where we know what the character's considering doing. But it should be there.