Having trouble understanding ていく

耳を傾けてくれていたエイナさんは、僕の話が進んでいく内に表情を険しくしていく。
I am so confused with the use of ていく in 険しくしていく and ていく in general.
I understand it in terms of physical motion as in, 持っていく, where you take something and then go, but I don't understand it's usage in other cases.
Going by the explanation on other sites, it is a movement away from the speaker?
So as the story proceeds, Eina is making a rough face away from the speaker?
I'm really confused, could someone please explain this to me?
Thank you so much!

～ていく/くる（きた）can express continuous action/movement in the time line. ～ていく is mostly for the future, and ～てくる/きた is mostly for the past.

Her face is becoming more and more rough/grim.

Thank you!