How do the actions of the secondary character you have chosen impact the eventual tragic ending of Romeo and Juliet?

I'm not sure which secondary character you have chosen, but I think that a good choice might be either the Nurse or Friar Lawrence.  Juliet's nurse is certainly very involved in bringing Romeo and Juliet together.  She disobeys the Capulets' implicit orders and serves as the go-between so that Romeo can communicate the marriage arrangements to Juliet.  She knows that Juliet's parents want her to marry Count Paris and that Romeo is a Montague, son of her parents' great enemy, but she helps the lovers anyway.  She enables them to communicate both before their wedding and after Romeo has been exiled.  She finds a ladder so that Romeo can climb to Juliet's room on their wedding night.  She hides Romeo's presence from Lady Capulet.  If she were more obedient to the Capulets, the tragedy could likely not occur.  

Further, Friar Lawrence is initially apprehensive about marrying the lovers because of the feud between their families.  However, Romeo convinces him, and thus begins his involvement.  Not only does he marry them, but he is responsible for constructing the plot where Juliet fakes her own death after Romeo's banishment, a step that leads directly to the tragic ending of the play as well.  Had Juliet not attempted this, Romeo would still be in Mantua, pining for her.  However, she would likely be married to Paris by now, and that would mean a different kind of tragedy.