Anselmo serves as Robert Jordan's guide in the area around the bridge, and introduces him to Pablo. Also makes the first indications that Pablo is probably not trustworthy.
After going back with Pablo and Robert Jordan to the cave, Anselmo takes Robert Jordan to view the bridge and determine how to blow it up. This gives him an opportunity to express his thoughts on killing. And to say that he's decided Pablo is definitely not trustworthy.
That night, when Pablo announces he is against blowing the bridge, Anselmo offers to do it with Robert Jordan alone – this may just be a ploy to shame the others to join in.
The next day, Anselmo is given the task of going to a spot on the road and recording all the fascist vehicles and equipment he sees pass by. He's to stay until relieved.
Taking that last bit very seriously, Anselmo stays out there long after a raging snow storm has started up, and is very cold. But we do get some quality time in Anselmo's head.
Robert Jordan eventually comes to bring him back to the cave, and offer him some "fire water." Anselmo likes fire water.
The following day, Anselmo goes with Robert Jordan, Agustín, and Primitivo to man the machine gun in case a fascist patrol comes too close to the cave. He doesn't actually get to do much, though: just gets sent back to the cave for an axe to cut some trees for camouflage, and, after cutting the trees, doesn't get to use them.
Robert Jordan wants Anselmo to observe the bridge again that afternoon, but Anselmo suggests he go to La Granja to gather what rumors he can.
On the way to La Granja, he sees pass by the fascist cavalry carrying the heads of El Sordo's band, and turns back to tell the others what has happened.
Presumably, at some point he goes back to his original spot near the bridge to record the movements of those pesky fascists. We don't actually get many details on this – we just know it happens, since he makes a report to Robert Jordan later that night.
Anselmo's report suggests the fascists are quite prepared for the Republican offensive, so Robert Jordan decides to send a dispatch to Golz. He asks Anselmo to take it at first, but on Anselmo's suggestion sends Andrés instead.
On the next day, Anselmo serves as Robert Jordan's partner in rigging the bridge while the others take the post. He has to kill one of the sentries, which is very hard for him.
Anselmo is killed by a piece of shrapnel which impales him when the bridge explodes. Shortly before the explosion, the narrator lets us know he was calm and at one with everything around him. (Is that foreshadowing?)