Jessica,
Unless you are a serious student of human behavior, skip it! The Feria de Abril is indeed a part of "Seville for what it is", but that is anything but charming, soulful, etc. First off, and just to disabuse your notion of Sevillian "openness", the Fair is for and about Sevillians. If you don't know locals enough for an invite, you won't be allowed access to the tents where the partying takes place. (There are a number of public tents, which are absolutely mobbed with "ambience".) This leaves you with the option of wandering the makeshift dirt streets between the tents, listening to the music, watching the Japanese tourists (who can't get into a private tent either) dance, avoiding puddles of urine and vomit, and hopefully not getting involved in a brawl between groups of drugged teenagers. This will be an experience, all right, at least until it starts raining. (Traditionally, Feria is the time for the annual Spring rains and a garbage collector's strike.) Feria is a shining example of Seville at its absolute worst. Even if the prices weren't abusive, you would do well to exclude it from your itinerary.
On the other hand, all towns have their own fairs. Just check the local paper or ask a couple of locals and they'll direct you to the nearest little town, where you'll get a perspective of Spanish generosity and festivity. The web www.fiestasdeandalucia.com/provincia.htm also has a partial list of fairs. (My personal recommendation would be Cordoba, at the beginning of May.)