The Janitor from Scrubs only interacted with JD in season 1, because the writers were planning for a big reveal that he was just another figment of JD's overactive imagination. Turns out Niel was just too funny, and they dropped that gimmick so he could also screw with the rest of the cast.
Gus from Breaking Bad was only a small character of little consequence. Giancarlo ended up being do captivating in his limited screen time that they had to change course to give him a larger role.
The neighbor in Home Improvement, Wilson Wilson Jr., was initially only ever spoken to by the main character Tim. Just like with Scrubs, they were planning a twist where it would be revealed in the finale that his house had been abandoned the entire time and the man Tim was leaning on for advice was either a ghost or a figment of his imagination. Again this was scrapped because people really liked him character and wanted to see him interact with the rest of the cast.