Life is a Dream was originally published in the year 1635 and written by Pedro Calderon de la Barca. It is a complex story that frequently blurs the lines between reality and dreams. There is a theme of free will versus fate, and a central struggle is the relationship between Segismundo and his father.
Segismundo is an interesting character, and I think a difficult one to cast. Modern day doesn't make a difference in the actor's abilities. The actor needs to be able to sell an audience on prison misery. Segismundo is in prison, because his father placed him there out of fear that Segismundo would one day be a terrible king. That is when the audience first meets him -- in prison.
Segismundo's father, Basilio, later invites Segismundo to the palace in order to find out if placing Segismundo in prison was the right decision. Basilio's concerns are supported at this point, because Segismundo acts very base. He's vile, crude, and even violent in the opulence of the palace.Segismundo is returned to prison, drugged, and told it was all a dream.
By the end of the play though, Segismundo is freed from prison and placed in power. He is not the same man that he was the first time. He is calm and gracious. His actions show a great maturity.
As for an actor that can pull all of that off, that's a tough ask. The actor needs to be younger. Not a young boy, but can't be old. He is the son of the current king and needs to be young enough to act brash and impetuous. The actor also needs to be able to pull off depravity as well as a regal air. My suggestion is Christian Bale. He's obviously capable of pulling off calm and regal. He plays a fine Bruce Wayne. He does cocky and brash quite well too as evidenced by his performance in "The Prestige." Bale is a chameleon and is willing to go to great lengths to change his body to take on a character, which is what he did in "The Machinist." I think Christian Bale is a great choice for a modern Segismundo.