Actor 'Nicolas Young (I)' confessed in the DVD commentary that he got the role of "John" because he cheated. He was working part time for his agent at the time and when he saw the breakdowns for the role of "John" and thought he'd be perfect for that role. Nicolas was responsible for sending over the head shots and resumes of several actors to be considered the role of John. So, Nicolas would send 1 head shot and resume of each actor (his competition for the role he wanted) and 12 of his own head shots and resumes as well. Although that's in the gray area of ethics - it did get him an audition and 2 call backs. The directors weren't too keen on Nicolas, but seeing as he was the only actor over the age of 18, they compromised on their idea of the perfect "John" and eventually cast Nicolas for the part.
During the majority of the series, when the lab was located in the abandoned underground station, the control panel for the entrance was a prop that previously appeared in "Doctor Who" in the 1975 Tom Baker story Doctor Who: The Ark in Space: Part One (1975)). The prop was originally made for the 'Gerry Anderson' series UFO (1970), and other panels and control desks from that series can occasionally be seen in both "Doctor Who" and "The Tomorrow People."