realism - Did Keanu Reeves actually know martial arts when he acted for the movie "The Matrix"?

To add on to what Alenanno said:
This is the case for almost any movie. With any of these movies you're seeing choreographed fights, not actual fights. They're more dance than fighting, there's 0 or near 0 actual physical contact, little/no improvisation, it's all heavily planned out for visual effect. Each punch, kick, block, was planned and known to all participants, and practiced many times.
Many of these roles go to professional dancers and martial artists because they need much less training, they're physically flexible, and depending on their achievements, their renown as dancers/martial artists might bring in more viewers to such a film. Dance and Martial Arts training generally involves a lot of memorizing long lists of movements, flexibility, and endurance, which is all that's generally required to do these fight scenes, and it requires less of the films budget in order to prepare the actors to learn/do the fight scene.
But anyone in physical shape, not just martial artists and dancers, can do these scenes with training and practice.