In the movie "Meet the Fockers" does the word "Focker" sound really different from the four-letter word? I don't hear any difference. Do all the actors pronounce this word really differently from native American English speakers?
The surname "Fokker/Focker" is Dutch in origin. It is a real surname that real people have. It is pronounced /fɒkər/ in English. In these movies the name is not pronounced the same as "fucker" (/fʌkər/). Robert DeNiro and Ben Stiller are both native speakers of New York English and they most certainly distinguish the /ɒ/ (as in "cot") and /ʌ/ (as in "cut") sounds. If you thought it was pronounced the same, you simply mis-heard.
In the original movie in the series, Meet the Parents, there was a joke that main character's true name was "Gaylord Focker", which amounts to little more than a throwaway homophobic schoolyard joke based on the similarity (but not identicality) in pronunciation of the name "Focker" and the word "fucker". In the sequels Meet the Fockers and Little Fockers the joke was mined for more cheap laughs.
It's a joke because they sound very similar not because they sound the same. It's a pretty lame joke as it is, but if they were pronounced exactly the same it wouldn't be a joke at all.
In American English, the second letter of the four-letter word is pronounced /ə/, while in focker the second letter is pronounced /ɑ/ (as in bock).
In British English, the pronunciation of those vowels is, respectively, /ʌ/ (as in cup) and /ɒ/ (as in hockey).
Other words with a similar difference are muck and mock, duck and dock, ruck and rock, buck and bock.
[Reference: the New Oxford American Dictionary.]
Maybe the joke relies on the fact, that these words sound similar, although they do not sound the same. Yesterday I saw an episode of the British sitcom "IT Crowd". There one of the major characters has a date with a guy named Peter File. On several occasions people mix up this name with paedophile.
The root of these kind of jokes is often that people pronounce the word incorrectly and trigger a huge confusion.
Even if the surname Focker is pronounced correctly throughout the movie, when DeNiro discovers his daughters name is going to be Pamela Martha Focker, many people find this funny.