"star" probably isn't the word you mean -- but the lead actor.
Sure, it happens...but not all the time, it's not considered bad manners NOT to.
I know that Gerard Canonico was the first cover for Moritz in Spring Awakening. Near the end of Blake Bashoff's b'way run, Bashoff was being pulled from performances due to rehearsals for the tour, so Gerard was going on quite a bit (and took over the role) -- all scheduled in advance. Gerard 'called in sick' one of those last times so that Brian Charles Johnson could go on as Moritz before he left the production. (to very mixed reviews).
I believe it to be more common when there is a long standing relationship between lead and understudy. It's not "the standard".
If we're not having fun, then why are we doing it?
These are DISCUSSION boards, not mutual admiration boards. Discussion only occurs when we are willing to hear what others are thinking, regardless of whether it is alignment to our own thoughts.