The Flash movie has been through so many changes in writers and directors in the years since it was first announced, it's only natural that a cast member who was attached to one version might have hopped off the project since then. Such was the case for Billy Crudup, who joined up when Rick Famuyiwa was aboard to make the movie, only to bail as the various faces shifted behind the scenes. But as the film picks up some speed at last, he's back in negotiations to appear.
Crudup will reprise his Justice League role as Henry Allen, father to Ezra Miller's Barry. The elder Allen is in prison after being framed for the murder of his wife, and part of the plot of the Flash movie will involve Barry looking to save her from the actual killer, with his time-travelling efforts creating problems for the multiverse. Which goes some way to explaining how we'll see both Ben Affleck's Bruce Wayne/Batman in the movie and Michael Keaton's take on the character.
Andy Muschietti is the current director, working from Christina Hodson's draft of the script. Kiersey Clemons was attached to return as Iris West (Barry's better half and a character who will reportedly have more screen time in Zack Snyder's cut of Justice League), but there's no official word yet on whether she'll also be back for the movie as it stands.
The Flash (or whatever the title ends up as) is scheduled for release in June 2022.