I have a great idea for a commercial; do I just call a company and pitch my idea, or is there a procedure?

Answer 1

In general, in a wide variety of situations, the process you are using does not work. Mainly because there is no process and managing one-off ideas would require a staff of employess and would result in a bunch of disjointed, non-useable ideas.
Also if it is a big company they are most likely using one or two ad agencies, so you would have to contact them. If it is a smaller company you could write a letter to the marking VP and say you have this idea and would they like to discuss it.
Summary - don't waste your time. In general you want to apply your efforts were you can have more of an impact.
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Answer 2

what would you do if they said - thats a great idea, go ahead and create my commercial? Can you do anything about it? Do you expect to get paid?

Dont waste your time, there is no procedure. Marketing directors at major companies are flooded with ideas from the general public, and 99.9999% of the time they have nothing to do with the marketing objectives of the company. The MD's do not have any time to deal with this - which is why they hire creative agencies, who have the experience in this field and actually create commercials based on a thorough ananlysis of the advertisers needs and objectives.