Use INTERJECT_COPY_TRANS for everything except in the following case. You can use INTERJECT_COPY_TRANS2 when the original speakers is going to perform actions directly afterwards (because using I_C_T it'll accidently transfer those actions onto the NPC interjecting)
e.g.
If Bob is gonna interject into some state in Gary's dialogue where directly afterwards Gary gives you a key, you need I_C_T2 instead of I_C_T. If you use I_C_T you'll interject fine but then the command GiveItem("Key",etc,etc) will apply to Bob and not Gary. I_C_T2 does some funny tricks to make sure the original speaker (Gary) will give the item, not Bob. However, you shouldn't use it all the time, only when theres a chance this bug will happen.