I'm not sure if this is common knowledge but I was told a fact about this film by someone in the know at the pub the other night.
Apparently when Knock-Knock is in bed and getting home tutoring. Paul Fraser plays another teacher that helps him. They decided that it didnt work out though and so his scenes were cut. Only problem is Paul was in the final scene at the magic show! So what to do? They decided to digitally remove him!!! Which must have cost a hell of a lot more than Paul's acting fee!!!