If my players wanted to do something like that, I would allow them a Sleight of Hand check opposed by the targets' Perception to pull off the material component trick. [If I did it to my players, the rolls would be done in secret.] If the Perception beats SoH, then a Spellcraft check is allowed. [Again, secret if I did it to my players.]
As the posters above me have said in different words, the illusion interacts with the players in this case, and not the other way around, so there is an automatic Will save.
This is where it gets tricky. I would not allow the fireball itself to do any damage, because there are other illusion spells for that. However, the illusionist could concentrate to make it appear that the players avoided damage but are surrounded by burning whatever... walls, grass, trees, etc. I would require a concentration check for each mentioned "interaction" with the illusion: attempt to stomp or bat out a flame, attempt to injure a foe with a burning plank, etc. Failure on the caster's part to make the check or even do the check would expose the illusion for that target. If a player wanted, I would also consider a knowledge (nature) or other check (if the player can justify it) to see if the fire burns as it normally (or magically, or alchemically, or elementally) would. Again, this depends mainly on the circumstances.
I would also require the casters' allies to make the checks as appropriate unless they are aware of the casters' plans (told ahead of time, or if that's his/her usual modus operandi).