If a player dies and takes control over the aliens can you use the "Interruption" card against them? The card it states you cancel a "player" action. Is the person controlling the aliens still a player?
I'm assuming by "aliens" you mean "intruders" as the rules define "Intruders" as "the name give to the alien organism by the ship AI".
If so then this is covered in the Nemesis FAQ
Can an Interruption card be played to stop an Action of the Intruder player?
No, it can only be used against other Characters.