Changing between FK and IK in a shot
Inverse or Forward Kinematics? This is a question that haunts most animators. Some only use IK, while others claim that IK arms make the characters look like puppets pulled by strings. Both have pros and cons, and it's a good idea to take advantage of what they are best at.
Normally legs are in IK mode by default. That's because their main point of control is where the feet touch the ground, and IK allows the feet to remain fixed while the rest of the body moves. Making a walk cycle with FK legs would be something very difficult. Arms, by contrast, are normally an appendage of the torso and are held by the shoulders, rotating around them.
This setup with IK legs and FK arms usually works fine in most situations, ...