UNER SIEGE, with Steven Seagal, Tommy Lee Jones and Gary Busey. Directed by Andrew Davis,. At Broadway Mongkok, Windsor, Harbour City, UA Queensway, Broadway Tsuen Wan, Ocean. WHEN action hero Steven Seagal describes his character in Under Siege as ''very different'' you have to take it with a grain - no, a warehouse full - of salt. Casey Ryback is no different from any of Seagal's other one-dimensional heroes who prowl about killing the bad guys. This time the one-man chop shop does his killing aboard the US Navy's largest battleship, the USS Missouri. Ryback is an ex-Navy Seal consigned to the galley for insubordination - and what luck for the Navy, because a group of terrorists has taken over the ship to steal its nuclear arsenal. The leader of the terrorists, William Strannix (Tommy Lee Jones), is an ex-CIA assassin while his partner, Commander Krill (Gary Busey), is a military specialist, so both know how to run the ship and ward off attack. But Ryback is more than a talented potato peeler, and his new found ally, Jordan Tate (Erika Eleniak), is able to kill as well as she can strip. As predictable as this film is, director Andrew Davis' recreation of the claustrophobic atmosphere on a battleship effectively heightens the suspense so often missing in shoot-'em-up films. Under Siege has some genuinely exciting action sequences. Since Busey and Jones are actors of a higher calibre, they do their best to create characters more interesting than the usual psychopaths who square off against Seagal. Busey tends to go over the top - smiling maniacally and laughing like a rabid baboon - but he is entertaining. Jones is the best actor in Under Siege and it shows quickly, as his character's subtle humour and efficient cruelty mix well to make him both believable and, ironically, likable. With action, a buffet of corpses and the usual dose of martial arts, Under Siege accomplishes what it sets out to do: mindlessly entertain.