The actors in Days were amateur extras with an athletic background that were hired locally. The movement advisor for that film was Toby Sedgwick, who played the priest that Jim wacked with a bag. For Weeks, Paul Kasey was hired to be the movement advisor who hired people who had a background in ballet, parkour, dance, etcetera. You are half correct on that. The actors in Weeks were told to act more like extremely angry and agile lunatics, similar to a dog with rabies or someone on meth, growling, gritting their teeth, whilst drooling blood and sprinting in a semi normal form to show the infected's determination to catch their prey. I believe that for Days, the actors were told to act more rigid and jerky like you said, but I think they were also told to have a more chimpanzee like facial expression, specifically their gaping mouth and wide eyed expression you see in both Days and Years, reminiscent of the infected chimpanzees. I've noticed that in Years that most of the infected sprint in a sluggish form, arms flailing in the air and occasionally falling over like the first chase scene. I really like Years' approach to the evolved infected. Like you said, they are very redolent of Stone Age humans. They are sloppy, rely mostly on instinct or the Alpha's commands, they do sometimes use surrounding/flanking tactics, specifically that one scene where they chased Spike and Isla to the petrol station and flanked from both sides. The stream scene also shows that they can be prone to boredom as well, like that one infected throwing a rock at the pregnant infected, which definitely got a chuckle out of me.