Keanu Reeves performed about 95% of the film's stunts himself. The only stunts that he didn't do are the ones in which John Wick gets hit by a car, and the one in which he falls down the stairs during the fight with Cassian (Common).
To prepare for the role, Keanu Reeves trained for three months. His training consisted of judo, Brazilian jiu-jitsu, marksmanship and driving. Reeves had retained almost all the training from the original movie, requiring very little time get get back up to speed. The instructors commented that by the end of his training, Reeves could easily be the one training them, and his proficiency ratings were better than most seasoned professional soldiers.
The character of the Italian tailor is played by the actual costume designer of the John Wick films, Luca Mosca.
The DJ performing at the party in Rome is Le Castle Vania, who wrote music for both John Wick movies.