  We extend recent results on semi-classical conformal blocks in 2d CFT and
their relation to 3D gravity via the AdS/CFT correspondence. We consider
four-point functions with two heavy and two light external operators, along
with the exchange of a light operator. By explicit computation, we establish
precise agreement between these CFT objects and a simple picture of particle
worldlines joined by cubic vertices propagating in asymptotically AdS$_3$
geometries (conical defects or BTZ black holes). We provide a simple argument
that explains this agreement.
