Casilla made three vital saves as visitors Malaga marginally enjoyed the better of the chances - but Argentinian stopper Caballero prevented the hosts from nicking it at the end, making himself big to deny Sergio Garcia.
Neither side have been renowned for their attacking fervour this season and both went into the game after humiliating defeats, with Espanyol beaten 3-0 at Levante and Malaga thrashed 5-0 at home by Celta Vigo.
Therefore, it was to little surprise that the game took a while to get going.
The first clear opportunity did not arrive until seven minutes were remaining in the first half, but in the space of 30 seconds, either team could have scored.
First Malaga attacked down the left hand side and the ball was sent back across the box towards the unmarked Roque Santa Cruz - but the former Manchester City striker struck the bar from 18 yards out.
The ball remained in play and the hosts countered, Jhon Cordoba heading towards goal and passing to Sergio Garcia, but a Malaga defender accelerated back and got across to make the challenge before the striker had time to shoot.
Felipe Mattioni then got down the byline and crossed for Cordoba, whose header was cleared off the line by Malaga captain Weligton.
In the closing moments of the first half, Malaga hit the woodwork again. Fernando Tissone approached the box, barging an Espanyol player out of the way and put Eliseu through to the left of the goal, and the Portuguese's shot hit the arm of Casilla before rebounding off the near post from close range.
Then at the other end, Hector Moreno found the ball at his feet just yards from goal, but as he went to ground, was unable to steer it on target.
Twenty minutes into the second half, Casilla made two decisive saves in the space of a minute.
First the goalkeeper stopped a deflected effort from Sergi Darder by sticking out a leg when he had already dived the other way - and a minute later he pushed a long range effort from Vitorino Antunes out for a corner.
With time running out, Caballero came to Malaga's rescue. The hosts broke down the left and Juan Rafael put in a low cross to Garcia at the far post, but the Argentinian goalkeeper came rushing out to thwart the striker.
The result means Malaga move two points clear of the relegation zone - but they could find themselves in the bottom three by the end of the week, depending on results elsewhere.
Javier Aguirre's side, meanwhile, stay eighth, failing to grab their opportunity to climb up into the Europa League places.