Having watched Dr Strange 2, I was surprised at the small role that Michael Stuhlbarg had - he returned as Nicomedus West, a rival doctor from the first Dr Strange movie. He's an amazing actor, and has such a depth of character, but seems ill spent in the dr strange trilogy...unless they're playing a very long game with him.

In the comics, Nicodemus west was a college of Strange, who also travelled the world and learned about magic, but refused to practice it, later becoming a pawn of a evil pharmaceutical company. While the movies might follow a similar path, I kind of hope they might be hiding a darker turn.

In the movies, Nicodemus is always casting doubt on Stephen Strange, in the first movie he doubted his abilities and his morals, in the second he again questioned Strange on his actions in Infinity War/Endgame. It would very much be in the vain of the Marvel Devil to spend years causing Doubt to rise in the mind of its heroes.

Do what do you think? Is Mephisto secretly already been cast, and is spending his time being 'close" to the future/current Sorcerer Supreme and making him doubt himself?

edit - image for reference

possible Mephisto