Inactive to Active Demons?

Yello, I'm working on my DooM/Doom II WAD and I want my imp to have his back turned and when I step over a line(def), I want it to trigger the imp becoming active and turn and face me and then start shooting me in the face with his fireballs or whatever they are. Please help?

The following advices assume that you're using ZDoom UDMF map format:

To alert the imp to player's presence while he's looking away, use either Thing_Hate, or NoiseAlert.

If you want him to be completely idle and undamagable before the player crosses the linedef, flag him as DORMANT to begin with, and then when the player crosses the linedef, make it trigger an ACS script to call Thing_Activate on the imp, and immediately after that, call either Thing_Hate or NoiseAlert as I've told you before.