For a random nick, as well as including the option to specify a nick, this section

on 1:TEXT:!turn*:#:{ 
  if ($2 ison $chan) { 
    msg $chan $2 Your Turn! Type !truth for truth or !dare for dare. 
    set %turn.nick $2 
  } 
  else { 
    msg $chan $nick $2 is not on $chan - Choose another nick. 
  } 
} 

could be re-written as

on 1:TEXT:!turn*:#:{ 
  if !$2 {
    while $istok($nick $me,%turn.nick,32) {
      set %turn.nick $nick($chan,$r(1,$nick($chan,0)))
    }
  }
  else {
    if ($2 ison $chan) { 
      msg $chan $2 Your Turn! Type !truth for truth or !dare for dare. 
      set %turn.nick $2 
    } 
    else { 
      msg $chan $nick $2 is not on $chan - Choose another nick. 
    } 
  }
} 

BTW: It's !turn not !nick to pick the next player/victim

With the way the script is written, a command has to be entered for a new player/victim to be chosen, rather than it being done automatically after a response.