"Write a script that allows the uppertorso, left arm, right arm, and head to fluidly turn as the player presses a direction key."

-- local script 
local player = game:GetService("Players").LocalPlayer
local character = player.Character or player.CharacterAdded:Wait()
local humanoid = character:WaitForChild("Humanoid")
local upperTorso = character:WaitForChild("UpperTorso")
local leftArm = character:WaitForChild("Left Arm")
local rightArm = character:WaitForChild("Right Arm")
local head = character:WaitForChild("Head")

local function onMove(input, processed)
    if not processed then
        local moveDirection = input.Position
        if moveDirection.X < 0 then
            upperTorso.CFrame = upperTorso.CFrame * CFrame.Angles(0, math.rad(-45), 0)
            leftArm.CFrame = leftArm.CFrame * CFrame.Angles(0, math.rad(-45), 0)
            rightArm.CFrame = rightArm.CFrame * CFrame.Angles(0, math.rad(-45), 0)
            head.CFrame = head.CFrame * CFrame.Angles(0, math.rad(-45), 0)
        elseif moveDirection.X > 0 then
            upperTorso.CFrame = upperTorso.CFrame * CFrame.Angles(0, math.rad(45), 0)
            leftArm.CFrame = leftArm.CFrame * CFrame.Angles(0, math.rad(45), 0)
            rightArm.CFrame = rightArm.CFrame * CFrame.Angles(0, math.rad(45), 0)
            head.CFrame = head.CFrame * CFrame.Angles(0, math.rad(45), 0)
        end
    end
end

game:GetService("UserInputService").InputChanged:Connect(onMove)