Puck in Midsummer Night's Dream is an agent of change, but it is important to remember that he is directed to make these changes by his master, Oberon, and although therefore he could be played as a micheivous individual (and indeed his gleeful recounting of all the tricks he plays on people in Act II Scene 1 seem to reflect this side of his personality) he is only doing the bidding of his master, Oberon, the "director" of the play that ensues in the woods - close parallels with Shakespeare as the writer of the play there. Oberon does accuse Puck of willfully complicating the situation, but this is an accusation that he denies. Puck has either been played as a mischeivous prankster or quite a sinister force in the play, but he is definitely a commentator on the action and of course an actor in it too. He enjoys watching the chaos he creates, and one of the most famous lines of the play is "Lord, what fools these mortals be!", which reflects the stupid things we get up to when we are under the "spell" of love!
So, specifics: Puck affects the action of the play in the following ways:
1. He annoints Lysander's eyes with the love flower instead of the eyes of Demetrius.
2. He scares off the players during their rehearsal by giving Bottom the head of an ass and then lets him wake up Titania so she falls in love with him.
3. He then annoints the eyes of Demetrius so he wakes up and sees Helena.
4. After the ensuing fight and argument between the four lovers he leads Demetrius and Lysander off in opposite directions so they don't fight for the love of Helena.
5. Finally, he breaks the spell, so that Lysander is in love with Hermia again, Demetrius stays in love with Helena and Bottom looses his head of an ass. Lots of work!