Well,when we say that someone has good "台词功底"，it means he can remember all the lines and he knows how to express appropriately. It's a comprehensive quality for actor,and to some extent , someone has "台词功底" also means he is a good actor .

I would translate it to an actor's "line skills". An actor's ability or ways of pronouncing the words and delivering the lines. Basically, it is the ability to say the lines effectively by using their own understanding.

Direct translation 台词 lines on the stage. 功 skill, merit, achievement 底 bottom line Together it means "The basic oratory skills of an actor".

Yes, an actor's line delivery, that's a close equivalent.