The Clarkes in ANTR were 'generic' characters, representative of 2nd Class.in general, of the newly married couples on board, and of passengers killed in the water. There really was a young couple called Clarke in 2nd Class, but they were not newly-weds and the wife survived. The fictional Clarkes, incidentally, look and sound more like a 1st Class couple and are seen to be occupying a very fine stateroom far superior to most of those in 1st Class!
They're actually based off a couple from the book. Walter Lord described a " western couple" being approached by a crewmember to get into a boat. Someone overheard the woman say " not on your life, we started out together and we'll finish together". This is the same dialogue that you hear in the film.Of course someone may have more detail on who this couple actually was.
I can't recall what Walter Lord had to say about this incident, but his source was surely Lightoller's memoirs. The couple he spoke to on the boat deck were clearly American ("evidently from the Western States") and probably young (Lights described the woman as a 'girl') but he made no suggestion that they were a honeymoon couple or that he had any idea who they were or whether either or both of them had survived.
Its just common sense.When one reads a book and there is a quote coming from one or more people,and then you see the movie version and see these actors saying those same quotes,obviously they are based off those people in the book. Of course the screenwriter of ANTR read the book, came across these lines, and decided to build more involved characters around the cameo in the book.Therefore you end up with generic character representation like Bob said above.This can also be reversed, for example the Lord And Ladyship people in the movie are clearly based off Sir Cosmo and Lady Duff Gordon due to some of the dialogue they say.The same thing goes for Honor Blackman and her husband Robert who say things almost verbatim what Lucian and Eloise Smith said.Since these generic characters have more screen time than most other "passengers" I still wonder why the producers didn't just use their real names?
Using real names can lead to legal problems. Certainly this is why the Duff Gordons were not named in the ANTR screenplay or cast list. Even the Ismay character, who couldn't possibly be perceived as anybody other than himself, is not named. In the cast list he is 'The Chairman'.