In The Book Thief, are analepsis and prolepsis necessary for creating intradiegetic time in a story that begins in medias res?

The first step to answering your question is to define the terms:

Analepsis is a term used when the narrative of a fictional story moves back in time to before the beginning of the story: a flashback.

Prolepsis is the representation of a thing as existing in a form before it actually does. In narrative, it is a flashforward and usually takes the form of foreshadowing.

The intradiegetic level is understood as the level of the characters in the story. Thus, intradiegetic time is time as it operates for the actual characters as opposed to the narrator.

And lastly, a story that starts in media res (Latin for "in the midst of things") is a story that begins in the middle of the action, with the setup and exposition done later.

Thus, when a story begins in the middle of the action, flashbacks and flashforwards become necessary to create the characters' timeline. A good example of this phenomenon in The Book Thief is the story of Max Vanderburg.

Max Vanderburg is a Jewish fistfighter who hides in the Hubermanns' basement during the height of the Nazi regime. The very first time Max is mentioned in the story, it is as part of a flashforward wherein the narrator informs the readers that the story will be about "a girl, some words, an accordionist, some fanatical Germans, a Jewish fist-fighter, and quite a lot of thievery" (emphasis added). In a moment of prolepsis, the narrator introduces all the characters of the story, including Max.

The next time he is mentioned is in a flashback where Liesel thinks back to the time that Max "arrived on Himmel Street carrying handfuls of suffering and Hitler's Mein Kampf." Thus Max's story begins in media res with his arrival on Himmel Street. A few chapters later, the narrative jumps to a flashback detailing how Max escaped the Nazis and traveled to Himmel Street. Thus, analepsis is used to establish the backstory to his arrival on Himmel Street. Our narrator announces, "I have Liesel Meminger in one hand, Max Vanderburg in the other. Soon I will clap them together. Just give me a few pages."

As soon as the narrator "clap[s]" Max and Liesel together, another instance of analepsis occurs. Liesel and Max's meeting concludes part 3 of the book. Part 4 begins with another flashback to the youths of Liesel's adoptive father, Hans, and Max's father, Erik Vanderburg, during World War I. This flashback details their connection and further solidifies why Hans would want to help Max by hiding him in his basement.

Max's narrative begins in media res, but it is the flashbacks to his childhood and his father's youth, along with the narrator's flashforwards, that establish his timeline within the novel. His intradiegetic time is a function of the analepsis and prolepsis woven into the narrative.

How do analepsis and prolepsis create the timeline in The Book Thief, which begins in medias res?

Analepsis and prolepsis are Greek words. Analepsis is often called “flashback” and prolepsis is likewise called “flash forward.” Intradiegesis or intradiegetic time refers to the situation of a literary work in multiple time periods. “In medias res” is a Latin phrase that means literally “in the middle of things.” It is used in literature to refer to narrative that is not in strict chronological sequence.

An author could use either analepsis or prolepsis to indicate the multiple time periods in which the work is set; it is not necessary to use both. Authors often employ both concepts to show the effects of particular incidents on later developments or to indicate character development.

In a work that begins in medias res, most of the action may still occur within one period. In The Book Thief, the main time period is the World War Two years when Liesel is living with the Hubermans. In this novel, the main time period is not literally the middle of Liesel’s life, as she was then a child, but closer to its beginning. From the use of prolepsis (flash forward) in the narrative, the reader learns that Liesel survived the war—avoided Death’s taking her—and was reunited with Max. Flashbacks are also used to show what her life was like before being separated from her parents and joining the Hubermans’ household.