Let we have two CRM module one is Contacts & other is Potentials.

Now there is  a look up Customer name (list of Contacts).
There are three more fields like:-

Email
Cell
Street Address
Those fields are in Contacts module.
Those fields should auto fill when change Customer name lookup is changed instantly.

We want to change those values based on lookup instantly when user change it.
before saving.
Those fields are can be changed by user also.

Can anyone give a solution for this?