The characters of Katie (Amanda Walsh) and Gilda (Iris Bahr) appeared only this once and were cut from revised pilot. Katie was replaced by a different character, Penny, played by Kaley Cuoco. Gilda provided the inspiration for Amy (Mayim Bialik), a regular character introduced in season three. Her personality was also used for the scientist character Leslie Winkle.
This was the first pilot, which was declined by CBS, prompting Chuck Lorre's team to retool the show. The revised pilot was successfully picked up by the network (Pilot (2007)).
Much of this Pilot script was re-used almost entirely intact for the second attempt at the pilot. The most obvious differences are the Set Design & Set Dressing was completely re-designed, the 4 character dynamic was re-designed to be 4 male nerds with the new 5th female character re-positioned as a neighbor now living across the hallway. The 2 female character traits were made more sympathetic in the Penny (Kaley Cuoco) version, and the female scientist traits in the pilot were later re-configured among the male scientists, and later the female scientists. Sheldon (Jim Parsons) was also re-configured to be more insular and childlike.
James Burrows directed both the first and second (Pilot (2007)) pilots, but has not had any other involvement in the series.