  It was recently conjectured that the quantum complexity of a holographic
boundary state can be computed by evaluating the gravitational action on a bulk
region known as the Wheeler-DeWitt patch. We apply this complexity=action
duality to evaluate the `complexity of formation' (arXiv:1509.07876,
arXiv:1512.04993), i.e., the additional complexity arising in preparing the
entangled thermofield double state with two copies of the boundary CFT compared
to preparing the individual vacuum states of the two copies. We find that for
boundary dimensions $d>2$, the difference in the complexities grows linearly
with the thermal entropy at high temperatures. For the special case $d=2$, the
complexity of formation is a fixed constant, independent of the temperature. We
compare these results to those found using the complexity=volume duality.
