Thoughtseize + target

[Falcon182]

Do I choose a target when I cast  {Thoughtseize} or after it resolves?

[Pleeb]

You choose a player when you cast thought seize. After it resolves you look at their hand and choose a card then.

[Remillo]

To clarify -

You target the player when you cast it, like you would with anything else that targets.
At no point are you ever targeting a card in their hand.  You're just choosing one after they reveal.

[Falcon182]

Cool. Thanks!

[Rass]

To clarify -

You target the player when you cast it, like you would with anything else that targets.
At no point are you ever targeting a card in their hand.  You're just choosing one after they reveal.

So what if they choose an instant. We will use  {Lightning Bolt} . Could I cast the bolt and make the  {Thoughtseize} Fizzle

[Pleeb]

This is where I think you're going with this question:  they cast thought seize targeting you and you only have lightning bolt in your hand. In response you cast your lightning bolt and now have an empty hand. You are still a player and thus a legal target when thought seize resolves, so it doesn't fiZzle. He gets to look at your empty hand, choose a non-existant card and lose two life.

If that isn't what you meant please rephrase your question.

[MuggyWuggy]

Or you use have a flash in aegis of the gods and their spell can fizzle as you will no longer be a legal target

[Pleeb]

{aegis of the gods}