He described how actor Chris Evans was shrunk to portray the skinny Steve Rogers in a interview with Film Journal.
Johnston said: "The technique involved shrinking Chris in all dimensions. We shot each 'skinny Steve' scene at least four times.
"When Chris had to interact with other characters in the scene, we had to either lower Chris or raise the other actors on apple boxes or elevated walkways to make skinny Steve shorter in comparison."
Johnston went on to state that he was not that familiar with the character of Captain America prior to shooting.
He said: "I was certainly aware of the character but had not been a regular reader. I didn't see this as a disadvantage of any kind. I was able to approach the character with a more objective viewpoint than someone who would call himself a fan."
Johnston recently revealed that Captain America: The First Avenger features minimal shield throwing, stating that the move is saved for special moments.
Evans previously admitted that he was uncertain about joining the superhero film, which is released on July 22 in the US and July 29 in the UK.
Watch the latest trailer for Captain America: The First Avenger below: