If I have an idea for a certain product, What is the role of Marketing in each stage from the inception stage to the actual production?

marketing team knows how customers think and what attracts or alienate them, so from stage of inception, they can keep up your idea to bring it out to life in the best shape that will achieve sales targets.
they will be able to determine the shape , color , functions, according to the market targeted segment ( age ,gender, educational level,social class etc..).
they knows best where your idea fits and they'll work on it to best suit the market segment it is intended for.
they will determine the best time of the year to launch your product and where to launch it, will help on publicity and marketing your product to customers.
in few words marketing team will help you turn your idea into a tangible product and will plan how will you be able to make money out of it.

In the very beginning, when you are only considering the idea, the role of marketer is to study the potential market place for your product, defining the amount of customers it have, understanding their wants and needs.
After that the marketer will have to decide which market segment to target, including the selection of customers to serve (picking the potential share of the selected market), choosing the value of proposition (the core value of your product that will differentiate and position it in the market), defining the applicable marketing management orientation for the market.
Note that at this stage you might be advised to apply some changes to your product to meet market's demand (design, usability and etc).
Later marketer should provide you with an integrated marketing plan which consists of your product's marketing mix (four Ps) putting the marketing strategy from the previous step to work.
On that stage your product is ready to face the market and the competitors.
Next part of marketer's job will be a creation of strong customer relationships which requires strong understanding of CRM environment (Customer Relationship management), defining the customer value and satisfaction levels.
And finally the last part of the marketer's job is to capture the value from customers in terms of financial return which is the profit from your product's sales.
At this stage the marketer will provide you with the information of customer loyalty, share of customers, customers equity and etc.
for your particular product.
This process is called a Marketing Process which is a core value of the marketing we know today and absolutely starting point for any marketing study.

Marketing people exactly knows what actually a customer is looking for, so they provide useful insight right from the inception of an idea to the product launch.

Well, if you have idea for a certain product the role of the marketing department from inception to actuall production would be as follow.

 you have developed idea

 Idea testing/screening

Your idea will be tested whether its possible to follow (have opportunities)

 concept development

after idea teasting, if there are opportunites a concept of makiing the product will be developed bases on your idea. For example a car. Its millage effeicency. headlights , its colors etc.

Concept testing

After your concept has been developed this will be attested. Many concept testing companies do this job for you. but you can directly go to your customers for this concept testing. based on this you will add or less the features in your product according to customers or the company testing the concept.

Marketing strategy

After concept development and testing there comes marketing stragtegy, Marketing mix (prince. place. product.promotion)

Business analysis

there comes then business analysis. Such as when to achieve break even. how long you will be bearing loses etc.

Product development

if approved after business analysis. product will be developed and will placed in to stores. ( limited numbers)

market testing

after this customers' response toward our product will be analyzed by marketing team.

Commercilization

If this product got good response and you analyzed there are opportunites for you to grow you commecialized your prodcut and make it available for everyone.