So my fellow geeks, is there any way we can consider Joe Pesci's turn on the Snickers' telly ad as canonical to the LETHAL WEAPON franchise, or the Angry Man in Scorsese Films Like RAGING BULL and CASINO franchise? Probably not.
The idea that there is an established body of works that shape a fictional character and others that do not, has spread like Vegemite thanks to Our Beloved Internet. Her, nerds and geeks of every stripe will argue, for example, which movies or TV series about the Teenage Mutant Turtles are canon and which are not. In some versions of the story, Turtles mentor, Splinter is the mutated form of a man called Hamato Yoshi and in other versions he is the mutation of a rat owned by Yoshi.
I am given to understand that Peter Cushing's role in the 1965 movie DOCTOR WHO AND THE DALEKS is not canon, but is considered part of some kind of extended Doctor Who Universe. Science Fiction franchises like Star Wars and Star Trek, often have meandering strands of storylines going back through decades and covering a variety of media.
It has been many moons since I spent time arguing whether a particular piece of Intellectual Property was part of a franchise's canon, extended universe or perhaps its continuity. I feel the deathly hand of the Grim Reaper tightening whenever I am tempted.
An interesting sidebar to this nattering about canon, sometimes the continuity rests in a single actor playing one character; Kelsey Grammer played Dr Frasier Crane for approximately 20 years and two US sitcoms–CHEERS and FRASIER. However, the character of John Munch played by Richard Belzer, best known for LAW AND ORDER; SPECIAL VICTIM UNIT has apparently appeared in nine other US television shows. Mostly these are guest spots in shows like THE X-FILES. His other long-running appearance was in groundbreaking 90s cop show HOMICIDE: LIFE ON THE STREETS. Check the Wikipedia for more on the phenom that is John Munch.