I won't pretend I know all about it, but I'll tell you what I was taught by my university teacher back when I was doing my bachelor's degree in history a couple of years ago.

Basically, yes, people thought the war would be short, they didn't believe the conflict would get to the point that it got to where it's just fighting for years and years. I mean, the recent wars in history (for them) had been short: Franco-Prussian in 1870-1871, Russia-Japan war in 1904-1905, the Balkan Wars in 1912 and 1913... Basically, people were used to short conflicts, not like WW1.

As for the opinions of people, that is a huge, huge historical/historiographical debate, as to whether the people were consenting to war or they disagreed with it. Many french historians (Becker, Prochasson, Cabanes, Deperchin and Duménil) claim that the french people were totally willing to go to war. Some other historians (Rousseau, Cazals, Offenstadt, Picard, Rolland, Mariot and Loez) claim they were forced to go to war, didn't consent to it.

So basically it's hard to tell, because even historians don't agree on the subject...