how to retrieve the values of the fields in order to insert them into other fields of another model in an automated action?

example:

I have a model: customer (id, name, address ...)

and a model: invoice (id, customer ...)

and a sale model:

at a sale I create an action to fill the invoice automatically and I want the 'invoice.client' field to contain the value of 'customer.name'

how can I do? 

Thanks