Robert Downey Jr., best known for his role as Iron Man in the Marvel Cinematic Universe, will soon take on a darker role as Doctor Doom in the upcoming ‘Avengers. Doomsday’. While the star has now brought two great comic book characters to the screens, according to him, there are two other characters that are difficult to match.In an interview with CBR, RDJ was asked which characters presented the biggest challenge for Marvel Studios president Kevin Feige and filmmakers Joe and Anthony Russo when adapting Marvel Comics stories for the screen. Interestingly, he chose Captain America played by: Chris Evans and Doctor Strange played by: Benedict Cumberbatchas the toughest adaptation in the MCU.Sharing his thoughts on the same, Downey said, “I still feel that the two most difficult characters to get are [Chris] Evans created Cap.”He went on to explain, “And then, I think, [Benedict] Cumberbatch with Strange was like, ‘Are they really going?’ Because in comics, it works. And they hold these spaces of credibility in the real world, and credibility in the world of magic and sorcery.”Captain America, as a hero, is just a super soldier, with a high moral compass and superhuman strength, his shield is his only reason, while Doctor Strange introduces mystical elements and alternative dimensions that expand the scope of the MCU. Both characters eventually became major characters in the franchise.Evans portrayed Steve Rogers in several Marvel films before handing over the mantle to Anthony Mackie after the events of ‘Avengers. Endgame’, while Cumberbatch’s Doctor Strange becomes one of the leading mentor figures in the current Multiverse Saga.Both Evans and Cumberbatch are set to return as their characters Steve Rogers and Stephen Strange in the upcoming ‘Doomsday’, with Downey set to appear as the MCU’s next big villain, Doctor Doom.The trio is also expected to return in ‘Avengers. Secret Wars ‘set to conclude MCU’s Multiverse Saga’.