Zombies are often shown dragging one foot behind them. The clip below shows an example from Shaun of the Dead (2004). You can see the zombie dragging its foot at 2:06.
How is this practical effect accomplished?
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The easiest way to achieve that is to use an amputee. And yes, zombie-type movies use a lot of amputees as extras. It's just easier and cheaper than green-screening something like that. Attach a prosthetic, and they can hobble, crawl, get their legs bitten off, whatever you want with a little practical special effects.
If it was a main character played by an actor that you know is not an amputee, then they will use a green sock and create the amputation effect in post processing.