Here's some of the many, many signs that made this twist so obvious:

    We never saw Gellar talk to, or interact with, anyone other than Travis. And no one ever talked to him.

    James Gellar, a man who seemed to thrive on the worship of others and the love of hot Teacher's Aids, vanished three years ago. No one has seen him since. Not a soul.

    When Gellar burns his own arm, he doesn't scream or complain.

    In episode 2 of this year, Gellar never talked to the hostage and the hostage never looks at Gellar when he stood in the doorway.

    Gellar objects to Travis having dinner with his sister but is then "okay" with Travis taking a random waitress out on a date. Oh, and the waitress never looks at Gellar while he's in the diner or when she's tied up on the kitchen floor. Also, Gellar doesn't even have a napkin or utensils at the diner.

    Gellar is not afraid at all to walk around in plain sight of everyone in the day time even though he's a wanted man with his face plastered on every Miami newspaper.

    When Travis loses his nerve and can't capture a "whore," Gellar tells him that he'll be with him. Then, in the car, Gellar pushes down on Travis' leg, which makes Travis accelerate into the woman.

    When Travis tells Dexter that he didn't kill anyone and that Gellar did it, that was a huge cue to the audience. If Gellar killed every single one of the DDK victims, because Travis chickened out, that means that during the scene in the season premiere, when Travis approached the fruit vendor with a machete that he... What? Chickened out and said, "Wait a minute. Stay right there, terrified man. I can't do this. Let me run back to the car and get my friend. He'll kill you with this here machete."

    Also, the police suspected two killers; one sloppy and one meticulous. Gellar was the meticulous painter. But then Travis said that Gellar also killed everyone. So he was also the sloppy cutter. Which would make no sense.

    Gellar won't even pay vendors at a Farmer's Market. He makes Travis do it.

    The episode "Nebraska," which featured an imagined version of Dexter's brother Brian, clearly showed us that figments could stand there and do things, like eating and drinking. Also, when the figment side of the brain takes over, that person is shown doing the killing. As shown when we all initially saw Brian killing the motel clerk with the pitchfork, when it was Dexter doing it.

    When Gellar wants Travis to come back to the fold, he goes to Travis' sister's house. And then he says "Just let me in," as he moves to just… walk into Lisa's house. As if Travis would have ever let him in even if they were still partners in crime. And he did it so nonchalantly. As if he weren't a person confined to an old abandoned church and just went into strangers' houses all the time.

    Gellar was not afraid to pop up at Lisa Marshall's elementary school in broad daylight and attack Travis with a shovel. Also, not afraid to stare down at Travis when Dexter entered the church, risking being seen by Dexter. When Dexter turns around, Gellar is able to hide fast enough not to be seen.

    Oh, and also, Gellar is apparently able to scamper up to the second floor of a church and then leap out a window and double scamper away into the forest in a matter of moments. Dexter buying this kind of near-superhuman agility was also fairly bogus.

    When Gellar takes over fully, Travis is either out cold or sleeping. Another device established by Fight Club.

    Gellar somehow knew the hotel and room where Travis was hiding.