 When Jack kisses Rose and the Doctor goodbye, John Barrowman (qv)  made a special effort to kiss both actors in exactly the same way...  except on one take, when he kissed Billie Piper (qv) as usual,  then said his line to Christopher Eccleston (qv), started the  kiss, and didnt stop until they fell on the floor. (Source: John  Barrowman on the DVD commentary)