Excellent question, one which I did some googling to refresh my memory. In the comics, the Phoenix Force actually reached out to Jean Grey a short time BEFORE it physically bonded to her. I wonder if
20th Century Fox adopted that premise for explaining the appearance of the firebird in
X-Men: Apocalypse?
It could be possible they meant the fire bird as a preclude of the Phoenix Force's actual arrival in the upcoming film by having it extend a small portion of its essence to Jean beforehand at a time it sensed her desperation.
That would help explain the firebird appearing in
X:A but I may be giving FOX too much credit and it is another continuity flaw that the studio has committed (again). I am only happy they are finally getting around to introducing the cosmic entity itself, represented by Jessica Chastain, along with Jean to form one being--INSTEAD of going the pathetic sole 'omega' mutant route seen in
X-Men: The Last Stand.
Then again they may be introducing the character as something less than one of the powerful beings in the Marvel universe. That will suck if true. Apocalypse is not a cosmic being but FOX de-powered his character so it will not be surprising if they do the same with the Phoenix.
EDIT: I am
assuming Chastain is playing the mental image of Phoenix that only Jean can see and converse with. For all I know, Chastain is playing the female version of Mastermind or some other nonsense the film does not need in my opinion.
Keeping my fingers crossed regardless.
