Assessing what is already there
Before anything is quoted, the substrate gets checked: flatness across the room, deflection in the floor system, moisture in a slab, soundness of existing backer, and whether the framing under a planned heavy stone floor is actually rated for it.
A floor that deflects too much will crack tile no matter how well it is set. If the framing needs sistering or blocking, you hear it at the estimate, not after the tile fails.
Getting it flat
Self-leveling underlayment for floors that are out across a room, patching compound for local dips, and grinding for high spots in a slab. Large-format tile is held to a tighter flatness standard because it has to be.
Walls matter just as much. A wall that is out of plumb or bowed will show in every grout line and every mitred corner, so it gets shimmed, floated or re-boarded before tile.
Isolation and movement
Uncoupling and crack-isolation membranes let the substrate move without transmitting that movement into the tile. Over slabs with existing cracks, over plywood, over radiant heat, this is what keeps a floor intact through seasonal movement.
Perimeter and field movement joints get planned into the layout at the spacings the standards call for, and filled with sealant rather than grout so they can actually do their job.