"De quien" is correct, in my opinion. However, "del que" sounds more natural. The sentence "El actor del que te hablé" is a subordinada sustantiva de sujeto, and usually is introduced by "que" instead "quien", in despite of the object in this case is a person. I can't explain it better, sorry.

Hi Diana. As Javier told you, both possibilities are correct. There is even a third option that is also possible: "El actor del cual te hablé..." However, I have to disagree with Javier when it comes to the grammatical analysis. What we have here is a defining relative clause. In Spanish when a defining relative clause is introduced by a preposition ('de' in this case) you can use 'el cual' and 'el que' and, if you are talking about a person, also 'quien'. So, in this case, you have those three possibilities: El actor del cual te hablé se llama Roberto. El actor del que te hablé se llama Roberto. El actor de quien te hablé se llama Roberto. The second one, as Javier has already told you, is maybe the most informal, the commonest in everyday Spanish, but the three are equally correct. However, if you are talking about a thing or animal, you can use only the two first ones: El libro del cual te hablé se llama Cien años de soledad. El libro del que te hablé se llama Cien años de soledad. I hope I have helped you.