Learn Spanish (Latin America): El actor le recomendó que cambiara la iluminación. - The actor recommended that he change the lighting.

Because the main verb, recomendó (recommended), is in the past tense. When you report a request, command, or recommendation that happened in the past, the following verb must shift to the past (imperfect) subjunctive. If the actor were giving the recommendation right now (recomienda), you would use the present subjunctive (cambie).

Who is le referring to, and who is supposed to change the lighting?

The indirect object pronoun le tells us the actor gave the recommendation to "him," "her," or "you" (formal). Because the sentence says que cambiara, that same person is the one who needs to do the changing. Without more context, le could be any of those three people, though the English translation here assumes "him."

Could I use the infinitive here and say le recomendó cambiar la iluminación?

Yes, you can! Using an indirect object pronoun (le) plus the infinitive (cambiar) is a common and correct alternative in Spanish. However, using que + the subjunctive is very frequently used for reported commands because it clearly emphasizes who needs to perform the action.

I sometimes see past subjunctive verbs ending in -se. Is cambiase correct here too?