The 1920s setting of the Fantastic Beasts franchise is a great opportunity for younger versions of beloved Harry Potter characters to appear in their own stories. Most of them, like Jude Law as Albus Dumbledore, are only shown portrayed by new actors — except one.
The exception is Jamie Campbell Bower, who crosses over from his cameo role as a young Gellert Grindelwald in Harry Potter and the Deathly Hallows Part 1. Grindelwald only appeared as a moving photograph and as a menacing figure in a window in Deathly Hallows, but Fantastic Beasts brings him back in a flashback reflection in the Mirror or Erised.
When young Dumbledore looks in the mirror, which fans recall shows the viewer his or her heart’s desire, he sees a younger version of himself (played by Toby Regbo) forming a blood pact with young Grindelwad; therein lies Campbell Bower, acting as a pre-evil Grindelwald whose closeness with Dumbledore is apparent.
It’s possible that Campbell Bower will return in later installations of Fantastic Beasts to better outline the relationship between Dumbledore and Grindelwald, which according to canon had a romantic element.
Until then, he’s the only actor who can claim to be a part of both Wizarding World franchises. Nice flex.