Last year's [X-Men: Days Of Future Past](tag:203942) may have been the most well-received iteration of the X-Men cinematic franchise, but it was certainly missing some major players from the earlier films - reduced to cameo appearances or minor characters due to plot and cast restrictions.
"[X-Men: Apocalypse] is about how the X-Men truly started" said director Brian Singer, and it truly is. Going back to the early 80s in the timeline, [X-Men: Apocalypse](tag:1194267) sets out the events of 20-years prior to the first film in the franchise, 2000's X-Men. The upcoming film will chart the beginnings of the X-Men as a team within the new timeline created in Days of Future Past, so that means the reintroduction of some characters who have been downplayed thus far in the prequel films. In particular...
Jean Grey / Phoenix - The Cinematic Original
It's important to state just how damn central Jean Grey is to the X-Men team, something that I don't think really came across enough in the films she's appeared in (apart from X-Men: Last Stand, but she appears there primarily as Dark Phoenix, not Jean Grey).
Famke Janssen was the original Jean Grey in the first three X-Men films, and honestly I did not really like the way her character was written most of the time. (Sorry Janssen, you did good with what you were given.) I think a lot of it came from the fact that Wolverine is always at the centre of every X-Men film he appears in, and in relation to him she's usually portrayed as this unachievable love interest (which is in the comic canon so I guess I can't complain too much).
Anyway, the conclusion of X-Men 2 saw Jean sacrificing herself to save the rest of the team and she then returned in X-Men: The Last Stand as some approximation of Dark Phoenix, resurrected but unable to control the massive power within herself. She kills Scott Summers/Cyclops, kills Charles Xavier/Professor X and is taken away by Erik Lehnsherr/Magneto, to be used as a pawn in his plot. At the film's conclusion, after much pleading, Wolverine kills her at her request.
Depressing right? But - surprise! - the time altering events of Days of Future Past mean that, when Wolverine travels back to the present, she's still alive - the events having led to her death not having occurred in the current timeline. Janssen and James Marsden (as Scott/Cyclops) appear in a cameo at the end of the film and it's all very gushy, but nice to see them back.
Sophie Turner as young Jean Grey
Clearly Singer is a bit of a Game of Thrones fan. After casting Peter Dinklage as Bolivar Trask in Days of Future Past on the strength of his role as Tyrion, Sophie Turner - best known for playing Sansa Stark on GoT - will portray the young Jean Grey/Phoenix in X-Men: Apocalypse.
Turner says she was cast for her portrayal of the "dark side" of Sansa, and draws lines between the two characters. She describes the young Jean as an outcast amongst humanity, struggling with her power which is both a gift and curse. Turner also reportedly took archery lessons in preparation for the role, will we see Jean Grey wielding a bow?
"[Jean] struggles with her power and not being accepted in the human world. I think in terms of [Game of Thrones], that's kind of the way Sansa felt in that she really wanted to be a normal princess/queen and live a normal life -- and she doesn't, because of all this havoc happens. I think Jean, in a way, feels that."
Skirting the Canon Differences
In both the comic and cinematic canons Jean's power emerges at a young age and frightens her non-mutant parents, who turn to Xavier for help. Xavier attempts to contain her powers and takes her into the school, she later becomes a founding member of the X-Men team and falls in love with Scott Summers.
In both canons Xavier places psychic blocks within her younger mind to protect her from the powers contained with her subconscious. These blocks limit her power output until she's old enough to control them. In the comics Jean's incredible power comes to the attention of the Phoenix Force, an immortal, powerful entity which later bonded with her, increasing her power as the Phoenix.
In the cinematic canon the placing of the mental blocks are what created the split personality later emerging as the primal, uncontrollable Phoenix (nice going Xavier).
Hopefully this is something which we'll see a lot of in Apocalypse, as the maturing Jean struggles with both internal threats in the form of trying to keep her sanity as her powers grow, and external threats in the form of the God-like mutant Apocalypse reappearing on Earth to rule over it's inhabitants.