Eso quiere decir ¨to stand on a soap box¨, pues quienes las hacían justamente se paraban en cajas.

This is to reinforce what Henry Hinds has correctly said.
In effect, to stand on a soap box can be translated as "arengar" (Word that comes from "arenga", discurso pronunciado para enardecer los ánimos. )

Please, see the following text I got from the internet:

"So why "Soap Box?"


Traditionally, when you want to speak to the masses (more specifically, when you want to harass the masses with your own opinions), you stand on a soap box.

That's why Soap Box is the title of this newsletter -- we're blatantly handing you our own opinions. Although we're not trying to harass you. (Much.)

Let us know if you see any masses to annoy."


Regards,

Clarisa Moraña

Yo usaría esta palabra..

¿Porqué están tan contentos los americanos? Hugo Salinas Price
... Su aparente éxito les permite sermonear al resto del mundo sobre la necesidad de
transparencia, de sistemas de bancos más abiertos a dar información, sobre ...
www.plata.com.mx/plata/comHSP4.htm - 14k - En caché - Páginas similares

TME 138 - Informe del Milenio: Una visión del Norte
... mundial, los países del Norte han ocupado hábilmente el terreno de la moral elevada,
y desde su posición estratégica se permiten sermonear al Sur -sus ...
www.tercermundoeconomico.org.uy/TME-138/analisis01.htm - 16k - En caché - Páginas similares

I'm not exactly sure how you shoulkd say this. Perhaps pregonar or sermonear would work. The sense of this sentence is that, if you want to make a successful political film, you can't preach your own political beliefs. Otherwise you would alienate half of your potential audience. And since the film is really intended to be a thriller... just in a political 'setting' or 'theme', not to expound your own personal beliefs.

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