Considering the film's uses of 'pissed', 'bastard' and numerous 'bloody's it was passed with an 'A' (now PG) cinema certificate by the BBFC after the removal of one line of dialogue. This was "What you wanted me to do that night" and is said by Liz to Billy during their walk in the park. The line was later restored in all video and DVD releases.
The audience's first introduction to the character of Liz was shot on the street, verite style. More often than not, the turned heads of the passers-by are genuine as John Schlesinger filmed their reactions to Julie Christie.
John Schlesinger chose Tom Courtenay over Albert Finney who had originated the part of Billy Liar on stage, mainly because Courtenay was a less physically imposing figure than Finney and therefore more believable as a dreamer.
Just prior to his arrival on the set Finlay Currie lost his last surviving daughter. He was understandably distraught and asked for a day off to attend the funeral. According to director Schlesinger, Currie was distracted and had trouble with his lines.