Learn Russian: Я хочу, чтобы герой медленно вошёл в кадр. - I want the hero to enter the frame slowly.

This is the main rule for expressing wishes for other people in Russian! When you want someone else to do something, you use a construction like Я хочу, чтобы (I want that) followed by the past tense of the verb. Even though the action hasn't happened yet, чтобы always forces the following verb into the past tense.

Why can't I just say Я хочу героя войти like in English 'I want the hero to enter'?

In Russian, you can only use хотеть (to want) directly with an infinitive if the same person is doing both actions (for example, Я хочу войти means 'I want to enter'). Because the person wanting (Я) is different from the person entering (герой), you must split the sentence into two parts joined by чтобы.

Why is it в кадр instead of в кадре?

Because the hero is moving into the frame, indicating direction or destination. When the preposition в means 'into' (showing direction), it requires the accusative case. Кадр is an inanimate masculine noun, so its accusative form is identical to its dictionary form. If you used the prepositional case (в кадре), it would mean he is already standing still inside the frame.

What kind of entering does вошёл describe?