It all begins with an 'Advertising Brief', given to the agency
by the client. This consists of a few standard parameters: what
we're going to talk about (ie, the launch of our new four-door saloon),
what there is to say about it (ie, it removes stains twice as fast
as... etc, etc), and other key bits of info, or 'selling points'.
Think of it as giving the agency ammunition with which to work.
The client will also have some input into which medium is used (TV,
press, radio, direct marketing) but, in theory at least, the agency
should look at the client's budget and advise on which media are
most suitable. This isn't always as simple as you'd think. TV is,
of course, an incredibly expensive medium with which to broadcast
your message, and may well not be suitable for your target market
or the product/service you're selling. And with 'new' ways of getting
your message across appearing all the time (ambient marketing, 'viral'
marketing, so-called 'guerrilla marketing', the ubiquity of the
Internet) there's a hell of a lot of choices to be made. In practice,
ad agencies are good at making advertising: ask them how to sell
your product, and they're going to suggest what they do best. Besides,
why would they send you somewhere else? God knows, they need the
business.