It is unknown when or why Bellairs initially chose the play for his doctoral thesis, though the fact it involved Nero probably had precious little to do with the decision process:
[John's] field of specialization was Jacobean drama, and choosing an unedited or little-edited text and making a new and more scholarly edition of it is a time-honored Ph.D. project. ...there may have been something about it that made it an attractive subject from John's point of view, but somehow I doubt that the title character was it. Knowing John, I suspect it was more likely a superabundance of silly lines.Whatever his interpretation of the source text or however much of his own work was completed is now lost. Once he received the letter accepting Saint Fidgeta for publication, "he burned the microfilm...he was working on and never looked back. (That’s how he told it to me, anyway.)"
Years later ("about 1974") Bellairs was asked why he never completed his work on his doctorate:
He replied that it was a long story, beginning with the sudden death of his mentor at Chicago [quite possibly Robert Cecil Bald (1901-65), whose "Seminar on Researching Elizabethan Drama" course Bellairs took], but mainly because he was tired of the whole academic process. I know he was also sick of teaching, and by that time in his life he knew he was never going to return to it. He also recalled a remark of Frank O'Malley's to the effect that a doctoral thesis in English consisted of finding some obscure 18th century English author and blowing him up all out of proportion.”Bellairs ultimately decided the dissertation, and the doctoral requirements on the whole, were not as important as they once were and, with Saint Fidgeta on the horizon, made it official: he wanted to write.