It would be almost impossible to make every line of NPC dialogue interesting, especially when it needs to prompt player action. Sometimes it's okay, and even quite necessary, to have the NPC say colorless such lines as: "Over there!" or "Duck!" or "Drop the gun!" or "Back again?" In such situations, you just have to let the voice actor bring as much life as he or she can to the line. After all, in times of emergency or in urgent situations, people speak much less colorfully.
In other, less danger-packed situations, the ideal to strive for is to make NPC dialogue interesting, even when the NPC is giving crucial information to or prompting action by the player.
Now that this chapter has shown some of the challenges in writing NPC dialogue, and hopefully some of the solutions, let's complicate matters more (in a good way) in the next chapter, where we'll look at Dialogue Deepening Techniques.