The gypsy woman is a good example. She is only in the book for a few paragraphs, and being a caricature type of character really helps because readers will likely all have the same initial preconceived notions. She's mystical, helpful, and not helpful all at the same time.

Similarly, the crystal merchant is an extremely flat character. He's a good guy. He helps out Santiago; however, the crystal merchant is exceptionally depressing in that he has given up on achieving goals. He's exceptionally stagnant in his life, and he doesn't see a problem with that. It's his one overriding trait, and thankfully Santiago eventually sees a problem with that.

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.

"The Alchemist" by Ben Jonson is, I think, one of the most perfect comedies ever written. Kenneth Tynan, the veteran theatre critic, described is as "good episodic play ...bead after bead, the episodes click together upon the connecting string, which is chicanery and chiselry."

It's basically a farce with a very dark underbelly. A conceit is set up: two conmen con stupid person after stupid person and get money from them. Each gull sees a different set of disguises from the two conmen: a lot of costume changes, a lot of props changes, and a lot of running aroudn ensue!

Yet then Jonson gradually complicates matters by having the gulls come in unexpectedly. The conmen gradually lose their control over the plot, and the strings become harder and harder to hold on to: and their methods of keeping the con running get ever more complicated (so, when Surly tricks them into thinking that he is a Spanish Grandee, they have to hide him in a room in the house and arrange to marry him off to Dame Pliant in order to get his money and get him out of the way!).

In one scene in Act 4, most of the gulls appear at the same time, and the conmen have to really struggle to hold things together. And it is JUST at the point where they think they have managed it that Lovewit comes back from his holidays, and they have to rapidly evacuate the house - forgetting about Dapper, whom they have locked in the toilet, in the process!

It's a difficult play to read on the page. But see a good production, and you'll see that it really is an amazing comedy.

There are many aspects which make this play a comedy - the theme of "gulling" or deceiving was very popular in Johnson's times and the audience is able to enjoy the "fleecing" of the central characters of a number of stock characters, that are deliberately undeveloped and rather shallow. These characters have also been carefully chosen to represent a cross-section of London society, so all can enjoy and laugh at the way they are tricked.

Besides the coincidences and slapstick comedy that ensues, the real humour lies not just in the skill of the "fleecers" (Face, Subtle and Dol Common), but in the characters who are fleeced themselves. For it is ultimately they who allow themselves to be deceived by their greed. They project all of their desires onto the central characters and are therefore easily gulled. The one room as well which is where the action of the play occurs, changes for each "customer".

Of course, there is the obvious comparison between the willingness of the audience to be gulled in exactly the same way, as we believe what is set before us, no matter how unbelievable or ridiculous, expecting the "alchemy" of the theatre to transform it into reality. The last laugh, therefore, is actually on us.