Use IK when
A foot must stay planted while the body moves over it. A hand rests on a table, holds a cup, or pushes a door. A character climbs or crawls. IK keeps the end point fixed so nothing slides.
Use FK when
An arm swings during a walk, a character waves, a tail or ponytail follows through, or a head turns. FK rotations create natural arcs, because each joint rotates around its own pivot.
Watch for these IK problems
IK moves the end point in straight lines between keys, which can look mechanical; add breakdowns to restore arcs. Knees and elbows can flip direction when a limb straightens; most tools let you control the bend direction. Over-stretched limbs pop; keep targets within reach or allow stretch only where the style allows it.
A practical workflow
Block key poses quickly with IK for planted limbs. Switch free limbs to FK and polish their arcs. Where a limb changes from free to planted, for example a hand reaching then grabbing, use IK/FK switching or snapping at that frame.
Tool notes
In Spine, IK requires the Professional edition. Moho supports FK and IK with target bones and adds Smart Bones for clean joint deformation. Rive's IK constraint has a strength you can animate, so you can blend toward FK. Toon Boom Harmony Premium documents IK for cut-out rigs.