Mostow probably helped out Paxton in his first directorial effort with some uncredited work on the film. Sounds like their friends and Mostow directed him in U-571.
I just played the Bill Paxton Frailty commentary yesterday. In it, Paxton mentions screening the film for Mostow, with Mostow giving him some good suggestions for a few tweaks to make afterwords. I believe he mentioned screening it for Sam Raimi as well, for similar purposes.