�Hola, Nev!
Well, from my point of view, both examples (both tenses) have practically the same meaning in the sentences you gave.
�Yo estaba viendo la tele cuando mi pap� lleg� = �Yo ve�a la tele cuando mi pap� lleg�
In fact, �estaba� and �ve�a� are both �copret�ritos�. The difference is that the first one is used in a compound (continuous) tense, and the second one, as a simple verb, but I repeat it, both sentences have practically the same meaning.
Hope this helps, and again, sorry for the delay!!
�Hasta pronto!