One thing to add to Keith's good advice to really help sell the shot -- don't just focus on the fireball; also be aware how the fireball would interact with its environment. If you rewatch the clip you linked to, you'll see the fireball seems to cast light on the actors and throws off heat waves.
Walter Soyka, Principal
Keen Live, Inc.
Presentation, Motion Graphics & Widescreen Design
RenderBreak: A Blog on Innovation in Production