Johnny Galecki was first introduced as "Kevin Healy", but immediately became "David" from his second appearance on. The reason for this was that Johnny was appearing on another show at the time with the character name of "David", which is what Roseanne Barr wanted to call him. Broadcasting rules at the time prevented an actor from appearing on two different, non-related shows using the same name (unless it was their real/stage name). By the time Johnny made his second appearance on the show, his other job had ended, and they were free to call his character "David Healy", as originally intended.Roseanne later makes an in-joke in reference to this situation, when she is arguing with Dan and says, "David's not even his real name! Darlene made it up!"
Is Lanford a real or fictional city?
Lanford is indeed a fictional city. All the exterior shots are from locations in Evansville, Indiana.
Where is Lanford located?
Technically, nowhere. Considering that Lanford is a fictitious city, it really doesn't exist. If you take the information given on the show, it could exist in a number of areas. For example, the real cities of DeKalb and Elgin are often implied to only be an hour or two drive away. It is often implied that it is near Aurora, IL (Home of Wayne and Garth of Wayne's World), which is a real suburb of Chicago (Due west of the city). This would make sense as David and Darlene once made it to Chicago and back in the time they were supposed to be in school. These might give you the idea that Lanford is somewhere around North-Central Illinois.
However, it was stated on a few occasions that they were in Fulton County, IL, which is really more West-Central IL, and is adjacent to Peoria County. That being the case, it is more accurate to consider that the location and other speculation the result of inconsistencies from the writers.