They call it acting...

But I don't think they always get that... because well.. too often they start dating their love interests....
Jessie and Rorie for Gilmore girls, Now Jessie is with the Cheerleader which technically there is no love interest on the screen....
How about the couple that was on House and now are broken up...Or how about the bizillion other shows/movies where the people start dating. And whether they are single before that is up in the air....