When passive? When active?

The movie started seven minutes ago.

Please turn your cell phone off; the movie has started.

So it's active. I wonder when I must use passive and when active.

The projectionist started the movie.
The movie was started by the projectionist.

A projectionist works in the movie theater.

AlpheccaStarsThe movie started seven minutes ago.

Please turn your cell phone off; the movie has started.

But here you have both active too, without referring to any person who may have started it.
You mean we can use both active and passive for starting a movie.

The movie started seven minutes ago. (active, intransitive*)
Please turn your cell phone off; the movie has started. (active, intransitive)

The projectionist started the movie. (active, transitive*)
The movie was started by the projectionist. (passive, transitive)

* the verb "start"

shininggirlYou mean we can use both active and passive for starting a movie.

Yes. There are 2 different uses of "start."

"start" = to begin.
"start" = to set in motion.

Aha I got it.
Is it true for all verbs?
For example,
her knee broke.

Verbs can be classified in the following transitivity classes:

1) Transitive
2) Intransitive
3) Copular
4) Ambitransitive
5) Ditransitive
http://www.linguisticsgirl.com/english-verbs-copular-intransitive-transitiv

shininggirlAha I got it. Is it true for all verbs?
For example,
Her knee broke.

You are discussing ergative verbs. What you are talking about is not true for all verbs. It's only true of ergative verbs.

Here is a list of ergative verbs in English.