Usually I'm first asked to come up with a storyline (either alone or together with other creatives) which is used as a basis for the film. I then write the text used in the voiceover and which will also appear on-screen.
A few examples:
Philips Lighting design movie
Wassenburg corporate video.
Explanatory movie about what Brainport Eindhoven does.
Introduction movie for MasterColour Elite lamps. Wrote the script and also carried out interviews through Europe.