I understand why movie animals and bad guys do this, but it’s incredibly annoying. So the bad guy–the bad lion, or bad mafia dude, or bad robot, or bad super-villain, or bad alien monstrosity–sneaks up behind the good guy. One pounce, one shot, one punch to the back of the head, one tentacle up the ass, and the good guy is dead. But does the bad guy silently dispatch the good guy? No, the lion roars, the robot whirrs, the monstrosity hisses, the black hat sneers, “Hey Cody!”.
Then the good guy turns around, sees the bad guy about to attack, and has time to either pull out his gun and shoot the bad guy dead, or take a moment to savor the situation and then take off running, with the bad guy in pursuit.
In real life when a lion sneaks up on you from behind they silently pounce on you, and you don’t realize you’re being attacked by a lion until you feel the fangs puncture the back of your skull. In real life it’s a lot faster to shoot someone when you’ve already got a gun pointed at them at point blank range than it is to whip around and pull out your gun and shoot them.
In real life, if a lion really does walk up to you and roar, he’s not hunting you, he’s threatening you. And the last thing you should do is run, because lions can run faster than humans, and if you run you trigger his chase instincts.
It’s a very dramatic scene, because the audience sees the danger, but the good guy doesn’t, and so we have dramatic tension, which builds with the threat as the good guy realizes the danger, and then the release as the chase starts. But just once I’d like to see a lion jump on a guy out of nowhere and bite his spinal cord out.