The first characters in Naruto have both names and surnames, like Uzumaki Naruto or Hatake Kakashi.
Then, later, characters starts having only one name. Like Gaara, why does he have only one name ?

Kishimoto : For me, Gaara's name was supposed to be Kotarou Fuuma. But my editor strongly disagreed. I objected and said it was a name fitting for a sand ninja and that went on for weeks.

Maybe he did refused because of Fuuma no Kojirou ?

Kishimoto : Even today, I still don't know the reason for that. When I said "Then how a I supposed to call him ?"
My editor said "Gaara" like "An Ashura that only likes himself" ( That was the kanjis means ). I actually liked that so I kept it.
Then later he told me that was the name of a Ski Resort ( Gaara Yuzawa in japan. ) and That he just returned from the place.
I said "That's just bullshit !" but since I already took the name in the story and I liked it, I kept it.
Later on I stopped focusing on surnames and just went for how the name sounds and what the name means.

Which names gave you more work ?

Kishimoto : This is never easy. The 1st characters were actually the hardest for me. Especially when there are a lot to find at the same time.
When the Chunnin Exam started, I had to come up with names for ALL THE ROOKIES in just one week. I didn't know what to do.
I remember one time, when looking at a screw, I had an idea and gave his name to Neji. That does tell you how much my inspiration was lacking.
More than one name, it's finding at lot at the same times that troubles me.
Then I wanted to give adjectives to the Kumo Ninjas. Like Samui ( Cool ) or Darui ( Dull ). But I could never come up with names as cool as I wanted.
I later became a really hard job to find new names.