The characters in a film are essential to the composition of its universe. They are so not only because they are the vehicles through which actions unfold, but because they are also the ones who, acting and feeling, mediate the experiences of metamorphosis that the film spectator undergoes. In the case of O Sangue, in the passage from the screenplay to the filmed version, several characters were eliminated. One was Nino's teacher, who had a preponderant role as a calming presence in the scene of the uncle's first visit to Nino and Vicente's house. Some students, classmates, such as João, Hugo and Matilde, were also suppressed and disappeared too from the film one scene in which students have to read a composition in the classroom.  Also at school, the screenplay included a scene with a character simply called the orator.

In the document that contained the screenplay, in the second half, Pedro Costa attached 4 photographs of actors. The first two  are those of the main pair – Vicente and Clara – or in this case, the actors Pedro Hestnes and Inês de Medeiros. The other two photographs are those of the poets António Osório and Mário Cesariny, whom Costa wanted to play the role of creditors. Due to production issues and Cesariny's sudden illness, Pedro Costa had to change his mind, choosing the experienced Henrique Viana and Luís Santos for the pair. However, the homage to Cesariny's poetry remains in the film, namely with the line "In every street I find you in every street I lose you", that Zeca sings as he passes by Clara and Vicente in the park sequence [54’]. It is to be noted that on the pages attached to the screenplay, one can read, next to the image of each poet, one of their verses, which would probably serve to inspire Pedro Costa during the shooting.

It is also important to emphasise that the choice of actors is a very delicate process, as the adaptation to an idea that the author has for a character is not linear: this connection can be in the walk, in the look, in the voice, in a greater identification with a personality type, etc. The hypotheses are endless: Pedro Hestnes's fragile and determined look, his slow and measured way of speaking, seem to reveal more completely this fine transition between a character described on paper and an actor of flesh and blood; between a figure, a physical outline, a profile and a real person.

Closing the quartet of main characters we have, in addition to Vicente and Clara, the father, played by the actor Canto e Castro, and Nino, who Pedro Costa chose after several visits to orphanages. Nuno Ferreira, the boy with the look of an adult, works as a kind of counterpoint to his brother: younger and, nevertheless, surer of his troubled process of growth. Reference should also be made to the role played by singer Manuel João Vieira, the character of Zeca who in the screenplay goes by the name Rajid, that of Nino's uncle, played by the great actor Luís Miguel Cintra, and finally Isabel de Castro, in the role of the woman who helps Vicente at the end of the film.