Marines interrupt
The Marine player can intervene in the Stealer action phase if any Marine has line of sight to a Stealer that just performed an action. The Marine player must still have CPs available.
The Marine player is allowed to perform one action (including putting a Marine on overwatch, or clearing a jam) with one Marine that saw the Stealer move. Only CPs can be spent on this action -- the Marines' APs are not left over from the Marine action phase.