I can imagine it must be a tiresome and tough task attempting to cast actors, that have similar visual and possibly mental attributes, to play our very favorite Marvel superheroes. "What if they don't pull it off? What will the fans think?"
But time and time again Marvel have shown just how on point they are when it comes to their masterful casting decisions. Whether the actors look completely different (which in a lot of cases they don't... Groot obviously included), or attempt to put a new spin on their character, in my honest opinion there hasn't been a single dud in the heroes of the MCU.
Or has there been? Take a look at the actors and the characters side by side with their counterpart and see for yourself: