Remember that the second you get hired to do work for another video company as a freelancer, you immediately become a part-time representative for that business.
How you handle yourself on the shoot will directly impact the success of the business that hired you. So be sure to be on your best behavior and bring your "A" game even more so than when you are shooting for one of your own clients.
Your job as a freelancer is not just to do great work for the other production company. It's to help them get more business! You should keep your eyes and ears open for any opportunities your client (the video production company) can pursue to get more business based on what happens while you are out on the shoot.
For instance, if you are doing a shoot at a corporation and you hear the Director of Marketing discussing that they need another video for a related product or service, you should call the person that hired you (if they aren't already on the shoot) as soon as you get a break to inform them that you overhead this information.
They will thank you for this and will most likely reward you with additional work in the future. This is truly a "I'll scratch your back if you scratch mine" type of scenario. Try to get a handful of business cards from the video producer before you go on the shoot so you can be ready to give them out when someone asks for it. Also, don't be afraid to collect business cards that you can mail or give to the video producer after the shoot. This will also be appreciated and should result in additional business down the road.
If you are shooting wedding that was booked by another videographer, don't even think about promoting your business at the wedding and reception. This is also a situation where you should have a handful of cards to give prospects who are interested in learning more about wedding video services.
Take down the prospects names and give them to the person that hired you. Be sure to tell the other videographer that you are available for those dates as well. If he/she books any of those weddings, they will feel compelled to hire you again. After all, you didn't just shoot, you also performed the task of generating hot leads which makes you golden.
When someone asks you if you work for "XYZ production company" (the video production company that hired you), your ego will encourage you to respond with...."no, I have my own wedding video business. I'm just helping out this other company tonight." You've worked hard to build your business so it will feel awkward to hold back on telling the world that you too have a video business.
Instead, you should respond with this "Yes, I work with XYZ production company often." If the person continues to pry until they figure out that you are working as a freelancer, they may be interested in working with you directly instead of hiring the company that hired you.
This is another point in time where it will be so tempting for you to bypass your client and to work directly with the prospect. BAD MOVE! If you do this you should be banned from the video production business for eternity!!!
Seriously though, refrain from this. It's very unprofessional and you never know whether or not these people know your client and are simply trying to stir up trouble. If someone wants to know more information about your business, simply give them your client's card and tell them that he/she can answer any questions they may have in the future.