The actor told the Los Angeles Times that viewers won't know if his character Emerson Hauser is a hero or a villain.
"Hauser is lethal, bad and dangerous to know," said the Jurassic Park star.
Asked if the character is good or bad, Neill replied: "I'm not going to tell you, because he's both of those things. [It] depends on where you are on the map on that day and where your moral compass is."
Alcatraz will follow detective Rebecca Madsen (Sarah Jones) and Dr Diego Soto (Jorge Garcia) - an expert on the famous island prison - as they investigate the mysterious reappearance of convicts who vanished from the facility years before.
Rookie Blue star Eric Johnson and Caprica actor Michael Eklund have both signed up for guest roles.
Alcatraz premieres on Monday, January 16 at 8/7c on Fox. The show will air on Watch in the UK.
> JJ Abrams Alcatraz switches showrunner after 'creative differences'
> Alcatraz filming delayed for reshoots
Watch a promo for Alcatraz below: