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Is the character using Dead Fall Talent (p.65 players guide) considered Knocked Down because he is laying?

Yes.

I’m not sure there is a rule that differentiates between being Knocked Down in combat and simply throwing yourself to the ground. As a GM I would rule that if a character is on the ground during combat they are considered to be Knocked Down and prone. Outside of combat being on the ground wouldn’t matter so much, say if you are just sleeping. Being Knocked Down has penalties but it also allows you to use the Jump Up combat option (which appears to be dependent on the character being knocked down).

So if a character successfully uses the Dead Fall Talent and fools his opponent I would let him use the Jump Up Combat Option the next round if they wished.