In what way does Puck’s spirit dominate the mood of the play in Shakespeare's A Midsummer Night's Dream?
One way Puck dominates the play is that he is the initiator of the actions that cause all the various confusions and problems. While he is Oberon's servant, executing Oberon's wishes, it is Puck who, for instance, administers the potion thus directly causing the problems. This trouble-making "spirit" of Puck's is well documented in Act I, scene i when he and Titania's Fairy discuss his exploits:
Thou speak'st aright;
I am that merry wanderer of the night.
I jest to Oberon and make him smile
When I a fat and bean-fed horse beguile,
Puck is the troublemaker of the night to whom trouble is a jest (joke) and to whom mischievous antics are stock-in-trade. It is this spirit of good natured trouble-making that pervades the play from beginning to end. For instance, when Helena and Hermia are arguing in the Forest in III.ii, they insult each other in the "spirit" of Puck, calling each other "puppet" and "canker-blossom."
What, will you tear
Impatient answers from my gentle tongue?
Fie, fie! you counterfeit, you puppet, you!
It might be said that Puck in some ways acts as a narrator of the play as he seems to be everywhere and involved in everything and, therefore, seemingly commenting on everything. It is this seeming omnipresence that facilitates Puck's spirit dominating the troublesome yet playful mood of the play.