I read somewhere that the character of the Janitor was originally only going to be a character in JD's daydream and that for the first season he didn't interact with anyone else?
Is this true? If so What made them change this approach?
From Wikipedia:
The role of Janitor was originally devised as a one-time gag in the series' pilot episode, Lawrence admitted: "When we watched the pilot, we knew instantly we had to keep this guy around."
so he wasn't even meant to be on the whole first season.
In season two he is also shown in the extended opening video credit. But that had to return to the short intro on request of NBC.
Yes, they purposefully set this up. If you watch the first season of the show, the Janitor never interacts with anyone except for JD. There are a few occasions when a janitor is interacting minimally with others, but if you examine those scenes, it's not clear that anyone else sees the Janitor there.
In season 2 and onward, they drop this shtick.
As revealed in the DVD commentary on several episodes, the Janitor character was initially to be used as a figment of J.D.'s imagination if the show had been canceled during the first season or the first half of the second. This would have been revealed to the audience in the finale. ... Since the start of season two, Janitor has had encounters with most of the other regular characters. He has even had an entire episode ("His Story III") devoted mostly to him.