Elemental Affinity

Elemental Affinity lowers the Action Point cost of spells by 1 when standing in a surface of the same element.

Elemental Affinity is a talent.

Overview

Elemental Affinity reduces the AP cost of spells by 1 (to a minimum of 1) when the caster is standing in a surface matching the spell's element.

Elemental Affinity is widely regarded as one of the strongest talents in the game for caster builds. The 1 AP reduction on every spell of a matching element allows significantly more spell casts per turn.
The talent is strongest for Geomancer and Hydrosophist builds, where creating poison, oil, water, or blood surfaces is straightforward. Standing on a poison surface reduces the cost of all Geomancer spells, not just poison-elemental ones.
The elf racial skill  Flesh Sacrifice creates a blood surface directly beneath the caster, providing an immediate trigger for Necromancer spells. The  Contamination spell then converts the blood surface to poison for Geomancer spells.
Pyrokinetic and Aerotheurge builds benefit less, as fire and electrified surfaces are riskier to stand on and harder to create reliably.
This is often the first or second talent pick for most caster builds.