2016-08-23 : Any enters-the-battlefield abilities of the copied creature will trigger when Stunt Double enters the battlefield. Any “as [this creature] enters the battlefield” or “[this creature] enters the battlefield with” abilities of the chosen creature will also work.

2016-08-23 : If Stunt Double somehow enters the battlefield at the same time as another creature, it can’t become a copy of that creature. You may choose only a creature that’s already on the battlefield.

2016-08-23 : If the chosen creature is a token, Stunt Double copies the original characteristics of that token as stated by the effect that put the token onto the battlefield. Stunt Double isn’t a token.

2016-08-23 : If the chosen creature is copying something else (for example, if the chosen creature is another Stunt Double), then Stunt Double enters the battlefield as whatever the chosen creature copied.

2016-08-23 : If the copied creature has {X} in its mana cost, that X is considered to be 0.

2016-08-23 : Stunt Double copies exactly what was printed on the original creature (unless that creature is copying something else or is a token; see below). It doesn’t copy whether that creature is tapped or untapped, whether it has any counters on it or any Auras and Equipment attached to it, or any non-copy effects that have changed its power, toughness, types, color, or so on.

2016-08-23 : You can choose not to copy anything. In that case, Stunt Double enters the battlefield as a 0/0 creature, and is put into the graveyard immediately (unless something else is raising its toughness).