  Let $(M, \varphi) = (M_1, \varphi_1) \ast (M_2, \varphi_2)$ be a free product
of arbitrary von Neumann algebras endowed with faithful normal states. Assume
that the centralizer $M_1^{\varphi_1}$ is diffuse. We first show that any
intermediate subalgebra $M_1 \subset Q \subset M$ which has nontrivial central
sequences in $M$ is necessarily equal to $M_1$. Then we obtain a general
structural result for all the intermediate subalgebras $M_1 \subset Q \subset
M$ with expectation. We deduce that any diffuse amenable von Neumann algebra
can be concretely realized as a maximal amenable subalgebra with expectation
inside a full nonamenable type III$_1$ factor. This provides the first class of
concrete maximal amenable subalgebras in the framework of type III factors. We
finally strengthen all these results in the case of tracial free product von
Neumann algebras.
