I know everyone has a different or near-similar take on Mulholland Drive, either way everyone has an opinion on it.
My theory on it has changed a few times, the very first time I saw it I had no clue what had just happened and I was very confused afterward. After reading up on the various theories of Mulholland Drive such as the classical "dream" theory and what not I looked for other things that people have suspected about the imagery in certain scenes, and what roles certain characters are really playing within "that" part of the movie.
My thoughts (not quite a theory at this point) works with the dream idea, in fact I support this more after watching the opening more carefully with the volume turned up; the camera angle of the bed and the pillow is the perspective of a person aka Diane. Not to mention that you also hear a person breathing pretty heavily so it is also largely implied that it is a person falling/about to fall asleep.
So here we go...
*Jitterbug Contest/Opening = Either a memory of Diane's that her subconscious would later draw upon to use as back story for her own character in the dream. I think the opening is either a memory of Diane's or she is asleep or half-asleep and when the contest is over is when she has left the living room and entered the bedroom to fall asleep (close-up of pillow/breathing)
*Limousine Ride/Crash = More or less the actual beginning of the dream sequence (earlier on the couch was like a daydream or a vivid/waking dream that Diane had because she was only dozing off due to being tired)
*Rita Character = Is a dream version of Diane, that is once the car crash has occurred Camilla is no more and we only have Rita who is in her own right more or less an entirely different person despite supposedly having amnesia (Rita's lack of memory is a result of Camilla being killed). Rita's reluctance to be seen by anyone (i.e. the laughing couple on the walk to Havenhurst from MD) can be seen as Diane's subconscious knowing that she has done a bad thing and the police are out to get her and so she must remain out of sight and unseen by others. Also earlier Rita hid from a cop car that sped by, I think can also suggest the idea that in reality Diane knows that she will be wanted by police for questioning and knows that she must keep a low profile.
Rita's personality is also drawn upon by Diane's own memories and hopes via her subconscious, because the real 'Rita'/Camilla was killed/is dead (and the dream/Diane knows this), a new persona is crafted to create Rita in place of the absent Camilla. I also think that Diane's guilt plays a huge part in the dream and is more or less directly responsible for the creation of Rita (using Camilla as a template).
*Aunt Ruth = I think Ruth's existence in the first half of the film has multiple meanings/uses. One of them I think is that Diane's own passage from Canada to Los Angeles was recycled by her subconscious in order to create a reason/excuse for Rita being at Havenhurst.
*Siren before Winkie's Scene = you can clearly hear a siren for just a second before the scene shifts to Dan & Herb inside of Winkie's. Sometimes in real life while you're asleep your ears can hear something like the TV, the phone ringing, a dog barking, etc but since you're asleep it doesn't always register as what it would be when you're awake. I think the siren (heard, but no cop car is seen) is Diane physically hearing the police coming down her street for questioning/to make an arrest.
*Winkie's Scene = I've read some theories on this that Dan & Herb are supposed to be secondary dream incarnations of Diane and Camilla or what have you. I've read that some think that the Bum is supposed to be a physical manifestation of Diane's guilt, this is what I think too - however I believe the Winkie's scene as a whole is like a makeshift "confession" of sorts. Nothing of murder or crime of any sort is mentioned by Dan, "this god-awful feeling" Diane's guilt for the hit taking over and crafting the various dream people as her subconscious/active mind sees fit.
*Adam in the dream = Another incarnation of Diane, this one I have viewed as the Diane leading up to who we know in the second half of the film. A lot of theories abound that she was a call girl/prostitute, might have been raped/molested, so on. I think that the dream Adam persona could be another variation of Diane herself reliving some experiences via someone else or dumping these negative experiences onto someone who has wronged her (or both.)
The scene with Adam catching his wife in bed with Gene I think can represent Diane, Ruth and the client/john/trick. Aunt Ruth comes home early from a trip and catches her niece in bed with an older man, someone whom she has never seen before and does not know. Furious at her niece's promiscuity she banishes her (and her john) from 1612 Havenhurst and onto the streets and later at the hotel I believe is where Diane lived (via her John) and began her career as a callgirl/prostitute because she no longer had her aunt to look out for her (Cookie informs Adam that men from his bank were there, access to his accounts and everything was frozen, he had been kicked out of his house - translates as Diane no longer having room and board as well as an allowance of money and protection through her aunt).



