In a recent talk radio appearance, Far Cry Primal cinematic director David Footman gave an interview where he spoke extensively about how Ubisoft took pains with world-building to create a Stone Age world like no other.
The interview, a transcript of which can be found here, covered a lot of ground. The first thing Ubisoft realized when they embarked on the project is that they were working against the sum total of all prior depictions of the Stone Age, and they needed to do extra work to make their version distinctive. The Stone Age depicted in Far Cry Primal is not 100 % historically accurate, or complete fabrication, but a careful balance between the two, to serve the purposes of making a video game.
One theme that defined Far Cry Primal is its fidelity to the time period, and Ubisoft was willing to commit to the not inexpensive research required. They did not just work with historians, but theater actors, linguists, etc, to get elements like the language and the movement of the people from that time period just right.
The performance capture acting became especially important for Far Cry Primal. Keeping the world accessible meant the characters needed to be easy to understand. Footman revealed they went out of their way to get theater actors. Great emphasis was placed on body language and gestures. They took pains to get the actors comfortable under the capture suits to get the best acting out of them, and even held multiple rehearsals to get performances just right. They had more rehearsals for these scenes than what actors usually do for television or film.
In the end, Footman says he hopes fans will enjoy immersing themselves in the unique and elaborately created Stone Age fantasy Ubisoft has built for Far Cry Primal.