When a player gets close to a NPC Have it's head follow them?

Constantly check a region3 or check every character. If a char is inside a radius then continuously cframe the head facing the char until the char leaves or the rotation is too great. This is a vague example for a script inside an anchored npc model. I hope you can use the math at least. Most importantly, you can face a different position by doing CFrame.new(myPos,otherPos).

local radius=20
local npc = script.Parent
local head = npc:WaitForChild('Head')

local function rotationDifference(oldCF,newCF)
    --self explanatory
    return (newCF.lookVector - oldCF.lookVector).magnitude
end

while wait(1) do
    --loop through characters
    local cf = head.CFrame
    local p = cf.p

    for _,player in pairs(game.Players:GetPlayers()) do
        local char = player.Character
        if char then
            local ppart = char.PrimaryPart
            if ppart then

                local pos = ppart.Position
                local isInRadius = (pos-p).magnitude < radius
                if isInRadius then
                    --found close character
                    if rotationDifference(cf , CFrame.new(p,pos)) < 1.25 then
                        --character is in front

                        --look at character until it leaves or goes behind
                        repeat
                            p = head.Position
                            pos = ppart.Position
                            head.CFrame = CFrame.new(p,pos)
                            wait(1/20)
                        until ( (pos-p).magnitude > radius ) or ( rotationDifference(cf , head.CFrame ) > 1.25 )
                        head.CFrame = (cf-cf.p)+p --old rotation
                        break
                    end
                end
            end
        end
    end
end

Okay. I understand what you mean by your question. First thing, have you even tried researching it? It doesn't seem like you looked for the answer.

Here, I have a few scripts that may be able to assist you. You would put the following scripts inside the NPC model.

wait(1)
follow = false
while true do
if game.Players.Player1.Character.Torso ~= nil then
    if game.Players.Player1.Character.Torso.Position.X - game.Workspace.Soldier.Torso.Position.X <= 10 then
        follow = true
    end
end 

if follow then
    game.Workspace.Soldier.Humanoid.WalkToPoint = game.Players.Player1.Character.Position
end
 wait(.5)
end

These use a Following Soldier NPC I made. I suppose you know how to do the rest.

local soldier = script.Parent
soldier.GunStorage.Gun.Parent = soldier

for _, script in pairs(soldier.ModuleScripts:GetChildren()) do
    if not game.ServerStorage:FindFirstChild(script.Name) then
        script:Clone().Parent = game.ServerStorage
    end
end

wait(1)


local AI = require(game.ServerStorage.ROBLOX_SoldierAI).new(soldier)


local DestroyService = require(game.ServerStorage.ROBLOX_DestroyService)

local function clearParts(parent)
    for _, part in pairs(parent:GetChildren()) do
        clearParts(part)
    end
    local delay
    if parent:IsA("Part") then
        delay = math.random(5,10)
    else
        delay = 11
    end
    DestroyService:AddItem(parent, delay)
end

soldier.Humanoid.Died:connect(function()
    AI.Stop()
    math.randomseed(tick())
    clearParts(soldier)
    script.Disabled = true
end)