When he cast Viggo Mortensen, director Joe Johnston (''Jumanji'') hadn't even seen the actor as Aragorn in the ''Lord of the Rings'' movies. ''If you can believe this,'' says Johnston, ''I cast him off 'A Walk on the Moon''' -- the 1999 drama in which Mortensen played a seductive blouse salesman. What made Johnston jump? ''You can see there's a dark side to Viggo.'' For Disney's survival-tale adventure, which pictures real-life horseman Frank T. Hopkins (Mortensen) participating in a 3,000-mile Arabian race on the titular mustang, the actor did most of his own riding on location in Morocco (where the key animal trainer got sidelined with dust-induced pneumonia, but not Viggo). Is Mortensen concerned about getting out of the gate with a horsey story so long after ''Seabiscuit''? ''I hate comparing,'' he says, ''but I think maybe you get to know the horse better in this story.''