Though several characters could be charged with encouraging the fight that takes place in act 1, scene 1, of Romeo and Juliet, by William Shakespeare, there are three who stand out as contributing to the fighting with specific verbal threats or actions: Sampson, Tybalt, and the crowd of spectators.

The scene opens with Sampson and Gregory, two servants from the Capulet household, discussing their hatred for the Montagues. As they talk, they cross paths with Abram and an unnamed servant, both of the Capulet household. Sampson is the first to encourage a fight, by telling Gregory, "My naked weapon is out. Quarrel, I will back thee" (1:1:34–35).

Moments later, though, he suggests that the law will look more kindly on the side who does not instigate the fight. "Let us take the law of our sides; let them begin" (1:1:39–40). When Gregory suggests that he will frown at the Capulet men as they walk by, Sampson says, "I will bite my thumb at them, which is disgrace to them if they bear it" (1:1:43–44). Basically, Sampson is trying to goad the Capulets into a fight by throwing out an insult that the Capulet servants cannot ignore if they want to sustain their honor.

Benvolio arrives and tries to break up the fight, but, in doing so, he appears—at least to Tybalt, a Capulet who is also just arriving on the scene—to be participating in the fight with a drawn sword. Tybalt threatens Benvolio. "Turn thee, Benvolio; look upon thy death," (1:1:68). When Benvolio asks for Tybalt's help in stopping the fight, Tybalt responds by stating his hatred of the Montagues:

What, drawn and talk of peace? I hate the word
As I hate hell, all Montagues, and thee. (1:1:71–72)

He then draws his sword and fights Benvolio.

The last encouragement for fight comes from a small group of citizens who are on neither the Capulets' nor the Montagues' side. Members of the group shout encouragement:

Clubs, bills, and partisans! Strike! Beat them down!
Down with the Capulets! Down with the Montagues! (1:1:74–75)

Ultimately, it is the entrance of Prince Escalus that breaks up the fight. He criticizes both families and the mob that has gathered there for what has become a routine disruption of the city's peace.

In summary, Sampson of the Montague house, Tybalt of the Capulet house, and a crowd of citizens offer specific encouragement for the fight that takes place in the opening scene. The fact that one instigator is from each family and the citizens are against both families sets up a theme of fighting throughout the play.

Source: Romeo and Juliet Folger Digital Text edition
https://www.folgerdigitaltexts.org/download/pdf/Rom.pdf