Cinematic or not, by RAW, it doesn't work, since the game does not define a time period between the moment when the fireball pellet leaves a caster's fingers and the time it detonates - which means it's effectively instantaneous. Since there is another fire-based spell, flaming sphere, which does have a defined speed, I have to presume that the lack of such information for fireball is intentional.
As per RAW, I believe it is possible to ready an action to move when the caster begins casting the spell, since this is permissible by RAW (i.e. you can ready to interrupt or counterspell). The casting itself takes one standard action, which is supposed to be a fraction of the 6-second combat round.
Extrapolating from the above, I'd house-rule that it is not possible to ready actions against spells with a casting time of "1 swift action" or "1 immediate action," or that it requires an opposed Initiative check or something.
Ad hoc, if the character were adjacent to something which could provide cover, I may allow him to use his readied action to dive for cover (with a successful Tumble or Jump check, whichever he prefers - or, in your case, a successful DC 15 or 20 Ride check).