Your experience of the war would vary greatly depending on a number of factors. I can't possibly cover all such variables, but I can give you some generalities.

On both sides of the war there was a great deal of economic and logistical dislocation and difficulty. Supplies were logically funneled to the soldiers at the front, but both sides experienced a good deal of difficulty in procuring and transporting essential items--food, ammunition, and the like. This was particularly true in the Republican zone, which increasingly faced hunger as a major issue. There would have been strong pressure to join some sort of military unit, but likely not the regular military. Instead, local militias were created by the various factions in the war. In the Republican zone, these militias could be Communist, Trotskyist (as was the one George Orwell joined), Anarchist, or others as well. In the Nationalist zone you were likely to join a Carlist militia or a Falangist (Fascist) one.

In either zone you might join up simply to cover your own opposite political leanings. You would join because sometimes the alternative was being the likely victim of a murder squad, and proving your credentials would at least give you a chance at living. There were holdouts of people in the other faction's territory--Nationalists in the Republican zone or Republicans in the Nationalist zone--but the danger was quite high if your political feelings were known before the war. Paul Preston cites nearly 200,000 people murdered behind the lines during the war, matching the number of soldiers killed. You could be killed for being a union member, schoolteacher, belonging to a political party, or being a relative of any of the previous groups if your region fell to the Nationalists. If you were in the Republican zone, it was dangerous to be a landowner, industrialist, monarchist, a member of a religious group, or even merely Catholic. The Spanish Civil War saw 6,832 priests and religious murdered--the largest such bloodletting since the Catholic Church's inception. (Jose M. Sanchez, *The Spanish Civil War as a Religious Tragedy, pg 8) The violence against noncombatants was ongoing and widespread in both zones--with militia units carrying out ideological purges behind the lines in both areas.

Paul Preston's The Spanish Holocaust is quite new, and quite excellent. It covers the ideological components particularly well, as well as incorporating recent research into the issue. The aforementioned Sanchez book is the best english language coverage of anticlerical violence in the war, and is remarkably unbiased given the topic.

Followup questions by OP and others are always encouraged.