Learn Russian: Плёнку нужно проявлять осторожно.

In Russian, the topic (what the sentence is about) usually goes first, even if it is the direct object. Since the sentence focuses on the film, Плёнку takes the first position. It remains in the accusative case because it receives the action of the verb проявлять.

The English translation uses a passive voice (needs to be developed). Why is the Russian sentence active?

While English heavily relies on the passive voice for general statements, Russian strongly prefers impersonal active constructions. Using нужно (it is necessary) + an infinitive + an accusative object is the most natural way to say that something needs to be done without sounding overly formal or clunky.

Why use the imperfective verb проявлять instead of the perfective проявить?

The imperfective aspect is used here because this expresses a general rule or standard procedure, not a single, one-time action. If you were telling someone to carefully develop one specific roll of film right now, you would use the perfective проявить.

Does the verb проявлять only mean to develop in the context of photography?

No, its broader meaning is to manifest, to show, or to reveal. You will frequently see it used with abstract nouns, such as проявлять интерес (to show interest), проявлять терпение (to show patience), or проявлять инициативу (to show initiative).