People make stupid mistakes

Your characters don't know what's coming next. They don't know that there is an ending already in mind for them.

And so, they are going to make stupid mistakes.

They don't know that their best friend is a killer. They don't know that last drink is one too many. They don't realize that they forgot to lock the door.

People make stupid mistakes, and your characters will too. Let them. It doesn't make them weak. It doesn't make them stupid. It just makes them human. And sometimes, it helps your story move forward.