Okay, so this one has two parts to it. 1) Donnie’s mom is someone who has experienced time travel herself. She was a student of “Grandama Death” who wrote “The Philosophy of Time Travel.” Because of this, she starts to see the warning signs in her son that he is exhibiting the signs of someone who is “out of place” with time. The Government is aware of this condition in humans, and actively goes out of its way to try and stop it. They lobotomized Grandma Death, and forced Mrs Darko to live a life in quiet suburbia, never to speak of what she knew ever again. This is why we see Donnie seeing things involving time travel around all of the other members of his family except for his mother. If you remember the final scene of the movie it shows the family crying and distraught over the death of Donnie. But his mother is sitting against the side of a tree smoking a cigarette. This is because she knew that Donnie would die, but was hoping to try and create another life for him by sending him to a doctor and keeping him on medication to try and subdue it. When she and her husband go to see the doctor she becomes very visibly upset by the idea that they have to put him on more medication. Not only because she knows it’s destroying a gift that she, herself, was forced to abandon, but also because she sees that things are spinning out of her control, and could eventually be leading to the horrific end that awaits her son. When it comes to the Government, they are the men who are the supposed airline agents (the men in the back can be seen standing with sunglasses on and looking very intimidating) and they can’t fully tell the house what happened about the serial number on the airplane. Why? Because they were the ones who tried to kill him. They decided to take out two birds with one stone, by using the airline his mother was on to kill him in his room. But it had to be the right time, at the end of Oct, when she would be on the flight. To monitor Donnie, the government assigned a temporal “watcher”, which is the fat, smoking man in the track suit, to make sure that he was proceeding along accordingly to be dealt with. This is the guy who is watching Donnie try to kiss his girlfriend on the path, and also the guy (in the same attire) to watch the house party on Halloween. The Rabbit (Frank) is an Alice In Wonderland reference, in that he was there to take Donnie through the rabbit/worm hole and away from the Government. Or he could just be a guide; someone that is there to help humans, at the very adolescence of their time travel, to continue on. He warns Donnie that his world will end in 28 days, and chooses the body of someone that came from the path of poor decisions as a warning. This being is formless, and exists only as conscious energy. That is why he asks Donnie “Why are you wearing that human suit?” So…kinda crazy, but what do you think?