In a conversation with the president of Marvel Studios, Kevin Feige, director Jon Favreau revealed that Robert Downey Jr. was set to play Doctor Doom in Fantastic Four (2005). However, the role of Victor Von Doom was eventually given to Charmed star Julian McMahon. 

I remember you had all met with [Downey] already for like Doctor Doom or something on another project. I think he had come through on maybe Fantastic Four, so everybody sort of knew who he was.

Favreau praised Downey’s acting and stated that he was the “puzzle piece” that completed the character of Tony Stark. 

I remember sitting down with the guy, and I was like, ‘He’s got that spark in him in his eye and he’s ready.’ That’s when we were in your office, and we were pointing to his headshot, saying, ‘We got to try to figure this out.’

After Downey was cast, the project’s popularity rose among other creatives and actors who decided to join. 

Once it was him, that’s when my life got a lot easier because he understood. He understood the voice of the character. And then one by one, people were just signing on board because now it became something interesting.

Check out the entire conversation in the video below.