How might "The Alchemist" be considered a comedy of humours?
I am assuming, by the Top Tags, that you are reading The Alchemist by Paulo Coelho and not the play by Ben Jonson.
A Comedy of Humours focuses on a character who exhibits one overriding trait, or humour, that dominates his personality. The idea surrounding the four humours got its start in Egypt, but it was the Greeks who linked them to the four seasons and the four elements (earth, air, fire, water). The idea is that a person's temperment was derived from an excess of one bodily fluid over another (blood, yellow bile, black bile, phlegm). The corresponding temperments are:
Blood which makes a person sanguine, or in modern terms, an artisan. Qualities include courage, hopefulness, and love.
Yellow bile is associated with being choleric or acting as a guardian, to use modern terminology. Qualities include being easily angered or bad tempered.
Black bile makes one melancholic, or modern terms describe it as being rational. Qualities include being despondant, sleepless and irritable.
Phlegmatic people are noted as being calm, unemotional idealists.
Now that the humours have been identified, I would have to say that Santiago would fall into the Blood humour. This is shown in his courage in leaving the seminary and then his sheep in order to pursue his Personal Legend. Throughout his long journey, he remains hopeful and shows love to others.