I can't imagine my favorite Marvel characters being played by different actors. For example, Natasha Romanoff and Yelena Belova wouldn't be the same if they'd been played by Emily Blunt and Emma Watson instead of Scarlett Johansson and Florence Pugh.

Sarah Finn, Marvel's casting director, is absolutely phenomenal at what she does.
What makes a lot of these actors so great in their roles is their love of the Marvel Cinematic Universe. For many of them, their casting was a dream come true — and it might not have been the first Marvel role they tried to get.
Here are 22 Marvel actors who originally auditioned for a different MCU role:
1. Dominique Thorne, who was introduced as Riri Williams/Ironheart in Black Panther: Wakanda Forever, originally auditioned to play Shuri in the first Black Panther.
The role went to Letitia Wright.
You can watch her screen test with Chadwick Boseman below:
2. Lupita Nyong'o, who plays Nakia in Black Panther, auditioned to play Nebula in Guardians of the Galaxy.
The role went to Karen Gillan.