In my experience they are extremely inactive, slowly patrolling the top 6" of the tank, sometimes just staying in the same place like a statue. Mine very rarely left the top 6".
But when I'd throw a worm in, he just came to life in a split second, nailing the worm before any other fish had even sensed the splash! Great viewing.
Unfortunately this wasn't the only time when he'd come to life. If you spook this fish they can go nuts, hence the need for a fair footprint of a tank, and a good lid!
As a footnote, I would have loved to have had a few of these but read of bad things about multiples fighting. But apparently you can get away with multiples if you get around five minimum. But of course you're looking at a big tank to house that many of these extremely fast fish.
Keep us updated on yours when you get it, and time will tell whether you have the macrolepidotus or the erythrurus!