The belayer has many duties beyond feeding out or taking in rope. A good belayer, when belaying either a leader or a second, will consider doing the following:
Belaying the leader, most important! If you're on the ground, spot the leader before (or even after) they make the first clip! No need to “belay” if there’s no gear in. This usually means the belayer drops the rope and stands with hands outstretched, ready to keep the leader’s head and upper body from smacking anything if they fall before clipping the first piece of gear. The moment the leader clips the first pro, the belayer drops their hands to the rope and starts the belay properly.
If you are with a new partner and top roping, ask how much slack is desired. Many beginners want you to keep the rope tight, while more experienced people will probably want a little slack.
Never pull the leader off by keeping rope too tight! Always gives them a meter or so of slack rope so they can move freely.
Keep the rope out of the leader’s way before the first clip.
Help build a multidirectional first placement, if needed.
Give encouragement to the climber, but avoid idle chatter. Keep your communication as short and clear as possible.
Tell leader about rope getting stuck in cracks or around horns (“flip rope”).
Warn leader (“grounder alert”) if they have climbed too far above their last piece of gear.
Tell leader about amount of remaining rope if it’s getting close to the end. Use the call, “feet 2-0”. (Most belayers underestimate the amount of rope left.)
Make sure the rope feeds freely. Flake the rope well, and watch for tangles. Tarps, rope bags or duffel bags are good for this.
Tell leader if they back clip (more of an issue when sport climbing, not a concern with long runners).
When belaying the second up to your stance, as the second approaches the anchor, the belayer tells them two things: 1) where to clip and 2) where to stand.