Which characters' actions are significant in "The Monkey's Paw?" Why?

This is a great request.  Many characters act in significant ways. 

First, Sergeant Major Morris acts in a significant way.  In fact, it is very difficult to determine whether he is a good or evil character.  He not only tells the tale of the monkey's paw, but he also allows Mr. White to take it, knowing what it could do and has done in the past.   So, we can say that Morris's action of...

This is a great request.  Many characters act in significant ways. 

First, Sergeant Major Morris acts in a significant way.  In fact, it is very difficult to determine whether he is a good or evil character.  He not only tells the tale of the monkey's paw, but he also allows Mr. White to take it, knowing what it could do and has done in the past.   So, we can say that Morris's action of allowing Mr. White to have the paw is a significant act. 

Second, Mr. White also performs a noteworthy act.  He takes the paw and makes a wish.  In fact, he makes three wishes.  This sets in motion the tragedy of the story - the death of Herbert, the possible revivification of Herbert, and his final going away.  Unwittingly, Mr. White proves that one should not tamper with fate.  

These two characters drive the story.  Without them, nothing would take place.