Surely Sam must have known how to "diassapear" in the task?
 

Any magicians assistant must be in on the trick to pull it off.

Sam didn't actually just disappear so he would have had to practice.

What if Dexter had picked a different housemate? Did they all practice the trick just incase?

Confusing.

It was a very simple trick. They screened him and the chair from view with the curtain and pulled both backwards when Sam was taken off and they then replaced the empty chair. Are you trying to suggest that Sam was in on the trick to deflect attention from Dexter. The look of surprised on Sams face and the look on Dexters when he was selected was awkward and surprising. Not traits shown by amateur actors, it was real and BTW Dexter knew what was going on, or at least he thought he did. Otherwise why would he have slagged Sam off if he thought he was going to return, please keep up.

Why are you so rude?

No need whatsoever!

Yeah, this. It doesn't take practice to be told to walk through an open curtain.