The challenge of animation (player character) lies in how the character reacts to real-time input, mo-cap in games is affordable (budget and engine).

Animators animate walk cycles in a straight line and any turns aren't animated, they are deformers that force the character to pivot on one leg and the whole body turning isn't really animated, but using some sort of in-game generated real-time IK while keeping the original straight line walk cycle. That whole process is the costly (engine) part. We can get an in-game screenshot to look photoreal; but when stuff move, that's where it all shows.

Those techniques are examples in FIFA and GTA I heard from friends who worked on them, not necessarily true for all games (especially low budget ones). Also, I'm not so much of an animator or rigger so something might not be completely correct.