Death's string quartet scenes
In Evangelion: Death, there are scenes of four young musicians who form a string quartet preparing for a rehearsal of “Pachelbel’s Canon”. The four musicians look and sound like Shinji, Asuka, Rei and Kaworu. Due to the impossibility of any of the pilots having met another pilot before the events of NGE (as they obviously do not know each other before having met during the series), the traditional explanation for these scenes became that they were simply artistic linking devices designed to show the events leading up to Instrumentality and nothing else.
It however makes more sense to actually assume that the musicians are real people in the actual NGE timeline. They are not Shinji, Asuka, Rei and Kaworu still, but they do exist nonetheless. The musicians share the voices, looks and perhaps even some personality traits with their respective pilot counterparts, but not entirely.
Further evidence for the musicians (who will henceforth be called Not-Shinji etc.) not actually being the pilots can be seen in the fact that the film itself refuses to link them. At other points in the film, characters are linked when the film jumps in time to meet up with the same people during different times and/or situations. For example, after a Second Impact scene with Misato, a scene follows with Misato and Kaji in bed. Before the bed scene, a title card appears saying: “15 years later, she was held by a man.” With the other evidence onboard, it’s obvious the film makes a connection between the young girl in Antarctica and the older woman in bed with a man, with further evidence stating they’re the same person. But after a frame of Unit-01 holding Kaworu in its hand, the film cuts to the first string quartet scene-and no connection whatsoever is made between Not-Shinji and actual Shinji. The film refuses to make those connections and it’s therefore logical to assume those characters are not actually related to each other.
Furthermore, none of the musicians have fully illustrated faces. They are only seen partially, against light. Not-Shinji’s face can be seen the most, Not-Asuka’s face is at its best hidden by red hair, Not-Rei’s face is never shown beyond the nose and Not-Kaworu’s face is never shown. The most likely explanation is that the faces wouldn’t have exactly corresponded to the faces of the pilots-especially within the cases of Rei and Kaworu.
The evidence that exists for the particular interpretation of the musicians being real people in the real timeline largely exists within Death itself. The title cards preceding the very first sting quartet scene give a very concrete time and place to the scene-18 months before the events of ep. 24, in Tokyo-2. As it would not make sense to give such a concrete in-universe time and place to a merely abstract linking device, the logical point would follow that the scenes did actually happen.
Furthermore, the supplemental source The Evangelion Chronicle: Side B says: "Before the Third Angel attacked, a quartet of boys and girls gathered in the school auditorium of the Third Middle School in Tokyo-2 to perform "Pachelbel's Canon". These young musicians bore an uncanny resemblance to the four Children upon whom the world's fate would come to rest".
Additionally, from the Refrain of Evangelion booklet: 1. Suiten for Violoncello solo Nr. 1 G-dur, BWV. 1007 1. Vorspier
This is the opening for EVANGELION: DEATH and an introductory part of Bach’s unaccompanied cello suite #1 played by a boy who looked just like Shinji.
Though supplemental sources can’t always be trusted, they should whenever what they say corresponds to what happens in the actual works themselves and in this case the supplemental source provided additional proof for the the theory.
Therefore, seeing all this proof, the most logical conclusion comes that the string quartet scenes showed real people in the real timeline who only look like the main characters of the show.
It bears worth pointing out that the possibility of the musicians simply being ghostly apparitions of a kind, not flesh-and-blood beings has been pointed out. However, since this theory has little proof to itself besides from mythological uses (and NGE doesn’t show many mythological characteristics of this kind), it is most likely not true.