Well first you would have to shape the mask to replicate the creature then add some animatronics
Probably, I doubt they'd change the actors.
Mammooty
yes, it is true the dog was actually pulled by a demon because they believed in these creatures and Katie was posessed because when she was young she believed in these creturesNo, it is fiction.
They usually change actors/actresses if they are not playing their role correctly. It may also be because the original actors/actresses have got another acting job somewhere else.
The actors playing the flying monkeys were attached by small cables to a system of pulleys, which gave the impression of flight.
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