Robert Downey Jr., best known for his role as Iron Man in the Marvel Cinematic Universe, will soon go a shade darker to play Doctor Doom in the upcoming ‘Avengers: Doomsday’. Although the star has now brought two major comic book characters to the screen, according to him, there were two other characters that were difficult to adapt to.In an interview with CBR, RDJ was asked which characters presented the biggest challenges when Marvel Studios president Kevin Feige and filmmakers Joe and Anthony Russo adapted the Marvel Comics story for the screen. Interestingly, he chose Captain America Chris Evans and starring Doctor Strange Benedict CumberbatchAs the toughest adaptation of the MCU.Sharing his thoughts on the same, Downey said, “I still remember what it was to pull off two difficult characters [Chris] Evans did with Cap.”He went on to explain, “And then, I think, [Benedict] Cumberbatch with Strange was like, ‘Are they really going?’ Because in the comics, it works. And they held these places 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 being his only defining factor, while Doctor Strange introduced mystical elements and alternate dimensions that expanded the scope of the MCU. Both characters eventually became the main characters of 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 has become one of the main mentor figures in the current multiverse saga.Both Evans and Cumberbatch will reprise their roles as Steve Rogers and Stephen Strange in the upcoming ‘Doomsday’, while Downey will appear as the MCU’s next big villain, Doctor Doom.The trio is expected to return in ‘Avengers: Secret Wars’ which is set to conclude the MCU’s multiverse saga.